Disclaimer: Harry Potter modulo seven is JK Rowling.

A/N: These chess game in this chapter uses the same endgame that was created for the film of Harry Potter and the Philosopher's Stone by International Master Jeremy Silman. None of that game made it into the final film in any way that the viewers could see, which is unfortunate because it would have made for a far more dramatic scene with the White Queen as a minor villain. With Ron showing off his chess skills in this story, I just had to include it here.

Chapter 20

"Professor?" Hermione said tentatively.

Professor Vector whirled around in shock at the two new first years who had come into the chamber. "Miss Granger! Mr. Weasley! What are you doing here? I told you to go back to your dorms."

Hermione paled under her teacher's glare and looked to Ron. "Uh, we wanted to make sure you were okay," the boy said nervously. "We were gonna go get help if something had gone wrong."

Vector rolled her eyes. "That was very thoughtful of you two, but very foolish," she said. "I'd dock quite a few points if we didn't have bigger things to worry about."

"The obstacles…" Hermione breathed. "Harry…?"

By now, she and Ron had both noticed their dark-haired friend glaring at them from Vector's side. "I told you so," he said.

"Oh, Harry, I'm so sorry." Hermione said. "I never imagined the Stone was in real danger. All the teachers were so sure their protections would work."

"But they didn't," Harry said plainly. He sounded very calm, given the circumstances, but Hermione could see the anger and fear just beneath the surface.

"It's not Miss Granger's fault, Mr. Potter," Vector said. "She made the best decision she could based on what she knew. Professor Babbling and I must both have made serious mistakes if our protections weren't enough to stop the thief."

"Do you think the others will be enough?" Hermione asked.

"I doubt it," Harry said.

"I hope so," Vector countered, "but unfortunately, there's no way to know, and no way to stop the thief since I can't get through this door."

"But this is your obstacle, isn't it, Professor," Harry protested.

"It is, but Professor Dumbledore has the only key. There's no way in without it."

"Well, then Snape must have found a way."

"We don't know who the thief was, Mr. Potter. But in any case, I still can't open the door."

"Well, do you know what went wrong?" asked Hermione. "Maybe we can replicate it."

"I have no idea. I thought I had guarded against all modes of attack. The statues certainly don't look physically damaged."

"What are they doing, anyway?" Ron asked, looking up at the statues that continued flashing colours between their huge crystal eyes.

"Well, they're sending pulses of light back and forth, so it must be some kind of code," Hermione said. She tried to think about what Vector was likely to come up with. "Does it involve factoring large numbers, Professor?"

Vector actually smiled a little. "Miss Granger," she said, "I confess my original plan was to build a combination lock for which the combination consisted of the prime factors of a six digit number…Then I met you."

"Me?"

"Yes. Your prodigious mental arithmetic inspired me to look for a more secure method of protecting the Stone. I contacted some fellow arithmancers and technomancers in America for advice and also began reviewing the muggle literature as best I could. It's not really the kind of material I'm used to, but I was able to figure out something useful called a "key exchange".

That was a new one. Usually, it was Hermione surprising Professor Vector with her knowledge, but Professor Vector knew about key exchanges? Those were only about twenty years old, even in the muggle world. That was pretty impressive considering most wizards thought muggle appliances ran on "eckeltricity". "Diffie-Hellman or RSA?" she asked.

"Huh?" Harry and Ron said at once.

Vector started. "Of course you would know about them. My obstacle uses a Diffie-Hellman key exchange."

Hermione nodded. The Diffie-Hellman key exchange was a way for two people to communicate a secret code even if all their communications were being monitored. It relied on some relatively simple mathematics that turned out to be almost impossible to reverse, so an eavesdropper who didn't have all the information wouldn't be able to reconstruct it. But even from her limited dabbling with computer programming, she knew that there was almost always a way around these kinds of systems. She started to get an idea. "So, how exactly does it work?"

"Well, since we don't seem to be going anywhere, Miss Granger…The two statues are Alice and Bob." Now that she looked past the crystal eyes, Hermione realised that the statue on the left was a female warrior—she looked rather like Joan of Arc, in fact. Ron and Harry raised their eyebrows at the mundane names. "Every five minutes, Alice flashes a random prime number, p, between ten thousand and twenty thousand, and Bob flashes a random generator number, g, between one thousand and p. The flashes are coded like an abacus—red, yellow, green, blue, violet, and white are zero through five, respectively—most significant digit first."

Hermione nodded again. "And then they each generate a random private key, raise g to the power of that number modulo p, and flash that number as their next message?"

Professor Vector's face lit up with pride, despite the situation. "Quite right—at least if I understand your terminology. Each statue chooses a random secret number—a private key, as you say—which must also be between one thousand and p. But the resulting exponents are repeated every ten seconds for the five minutes, and if something goes wrong, the cycle immediately starts over."

By now, Harry and Ron were looking back and forth between the two witches as if it were a tennis match. "So, is that some kind of advanced Arithmancy?" Harry asked tentatively.

"Is that even English?" Ron suggested.

"It's modulo arithmetic," Hermione said. "It's like…it's like doing math on a clock. You can add and multiply and stuff, but whenever you get to twelve, you have to start over at zero—except with a much bigger clock, and it's a lot fancier."

That was something of an understatement, Hermione knew. Professor Vector tried to explain it to the boys and did a little better, although it was hard, since they weren't used to the idea of letters standing in for numbers. The two statues shared two numbers, p and g, and they each picked a secret number, raised g to that power, divided it by p, and took the remainder. Then they flashed that number to the other statue, which raised g to the power of that number, divided by p, and took the remainder. When it was over, the mathematical rules meant that each statue ended up with the same new secret number.

"If you flash that new secret number with your wand, it activates the statues and opens the door," Vector finished. "But there's no way to calculate what that number is fast enough. The only way through is with Professor Dumbledore's key, which is connected to the statues by a Protean Charm."

Harry and Ron still looked pretty well lost, but it all made perfect sense to Hermione, except for how someone had managed to get through it.

"Well, the thief certainly couldn't have brute forced it," she said. "It would take a minimum of four hundred arithmetic operations to figure out what the code is. Even I can't do discrete logarithms that fast, even with a calculator."

"Then it ought to have worked, since besting your skills was precisely my goal," Vector said.

"Hmm…well, if he didn't brute force it, the next most obvious answer is that it was a man in the middle attack." Hermione stepped forward and stood in between the two statues. "He could have stood here, flashed his wand, and pretended to be Alice talking to Bob and Bob talking to Alice at the same time. Then he could just feed them the code he wanted."

"But I thought of that, Miss Granger. I hope you don't think I would be fooled by a simple man in the middle attack. That's why Alice and Bob only wait ten seconds to flash their exponents the first time—that's the fastest I could get the mechanism to compute them. Anyone who wanted to try such an attack would only have ten seconds to set it up."

Hermione's face fell. She worked through the procedure in her head and realised that even that would take twenty operations on four-digit numbers. There had to be a way to make it work, though. The thief had to have figured it out. Sure, maybe he'd just overpowered the statues with magic, but she felt like she was missing something—like there ought to be some way to make it easier—to do it with just a few calculations…

"Of course!" she yelled. "Harry, give we your wand."

"What?" he said. "Uh…okay?"

Hermione took Harry's wand in her left hand and ran back between the two statues.

"Miss Granger—" Vector started.

"Lumos!" Hermione cried. To her delight, both wands lit up. She pointed one of them at each of the statue's faces.

Vector stopped short, her eyes wide. Two-handed casting was no easy feat in general, and especially for a first year.

But Hermione was just getting started. Professor Flitwick had taught them how to make coloured flashes of light in Charms Class, but at the time, it had just been a fun diversion. Now, she actually had a good use for it. "Gules, gules, gules!" she said. Both wands flashed three pulses of red light, or triple-zero, which scrambled Alice and Bob and forced them to start the cycle over. Hermione got ready to do some of her fastest mental math, and ten seconds later, it started.

First, Alice flashed her prime number, p, with the sequence yellow, yellow, white, blue, white, blue, white, green, which Hermione immediately translated to 11,887.

Bob's number was next, the generator number, g: blue, violet, white, blue, yellow, or 3,481.

And then Hermione knew what she had to do. She had to square 3,481, divide the answer by 11,887, take the remainder, and translate it into colours, all in less than ten seconds. It was near the limits of her skills, but just in time, she managed it: 4,508. Keeping the two wands aloft, she yelled out, "Argent, Purpure, Argent, Gules, Gules, Argent, Azure!"

Time seemed to stand still as Alice and Bob processed this input. Even trying to wield Harry's wand left handed, the colours had thankfully come out right, but she had no idea if the statues would accept them.

Then, Alice and Bob flashed a new pair of numbers: their exponents. Alice flashed white, violet, white, yellow, green, white, yellow, or 9,626, and Bob flashed yellow, yellow, blue, white, violet, or 1,139. Of course, Hermione didn't know if those numbers were in response to her or to each other's numbers, but she had to try.

It was a hard task, though. She had to do the same maths operations as before twice, all while still flashing her original number every ten seconds. She did her best to ignore Harry and Ron muttering to each other about how they had no idea what was going on and Professor Vector watching her sceptically as ten, twenty, thirty seconds passed. Finally, just after the forty count, she had the answers. If she was right, Alice's password was 711, and Bob's was 1,638.

Hermione lowered her left hand and kept her own wand raised toward Alice, praying this would work, and said, "Argent, Vert, Or, Or!" Then, she lowered her wand and pointed Harry's toward Bob and said, "Or, Argent, Or, Azure, Argent, Azure!" She just barely avoided tripping over that last tongue twister.

Suddenly, there was a loud crack, and a grinding sound behind her. Professor Vector gasped loudly, and Harry and Ron started grinning. Hermione spun around and saw Alice and Bob lifting their crossed battle axes into an upright position, and the heavy stone door beyond slid open.

"Hermione…How…?" Vector said in disbelief.

"Easy," she replied. "I just used two as my secret number. It's not in the allowed range, but Alice and Bob wouldn't know that unless you coded in a billion-entry lookup table, or they could perform discrete logarithms themselves."

Professor Vector thought about how the maths on this worked out for a moment, and then hung her head. "You're right, Hermione. I admit I hadn't considered that possibility. But I thought I had also charmed the statues to only accept the light from the crystals for the exponents, so that attack still shouldn't have worked."

"Snape must've found a way around it, then." Everyone jumped, as it was Harry who said this.

Vector sighed. "While I'm still not convinced of the identity of our thief, Mr. Potter, I'm afraid I have to agree with you on that. Some sort of spell must have allowed him to feed in his own number, just like Hermione did—and it would take more than a simple Confundus to do that."

"Well then, what are we waiting for? Let's go," Harry insisted, starting toward the door.

"No!" Vector said. "I will go on and do what I can to stop the thief. You three need to go back at once and warn the castle. You're small enough—the broomstick should carry all three of you."

"No, I'm going with you," Harry said.

"You most certainly are not."

"I am, too!" said Ron, running up to Harry's side. "I'm not leaving Harry behind again. I'm sorry I didn't believe you, mate."

"Mr. Potter, Mr. Weasley, I am ordering you and Miss Granger to go back."

"Then you can expel us later, Professor!" shouted Harry. "No one believed us before. You were the only one who would even listen. We're the only ones who can stop the thief."

"They'll listen if you give them a good reason. Miss Granger, go to Professor McGonagall's apartment—I assume you know where it is—and tell her we reached…" Professor Vector stepped forward and looked through the door, trying to make sense of what she saw. "Yes, tell her we reached the chessboard. This will be her obstacle. She'll listen to that."

"Chessboard?" Ron said excitedly.

"We have to play across?" asked Harry.

"Almost certainly. That's just the kind of obstacle she would design."

"Then you're gonna need Ron," Harry said.

"I doubt that Mr. Potter. You haven't seen Professor McGonagall play chess. It'll be hard enough for me to beat her. And if you remember your lessons, the transfigured pieces will be as smart as she is."

"You haven't seen Ron play chess, ma'am. You need Ron. And I'm coming with him. Hermione, are you in or out."

Hermione went pale as all eyes turned to her. Should she continue to be the voice of reason? But things had gone so mad already, what was the voice of reason anymore? "I…" she started fearfully. "Ma'am, we can always go back without any trouble, right?" she asked. "We could go for help anytime?"

"Yes," Vector admitted. "Professor Dumbledore insisted that we not allow anyone to become trapped down here."

"And he was okay with the 'very painful death' at the start?" Ron demanded.

"Well, it's not that hard to escape," Hermione said. "We got away from it the first time."

"When did you…" Vector started. "Never mind. I don't want to know. Look, I don't like this, but we're going to have to reach a compromise. I have to go on, and quickly. I can't force you three to go back because I won't be able to get back through this door on my own. There should be three or four more obstacles, including the chessboard. I'll permit you to help me work through those if you can, but I want you to promise me that if we run into the thief, or if things become too dangerous, you'll go back immediately and call for help."

"I promise, ma'am," Hermione said at once.

"Yeah, Harry and I will, too," said Ron.

Harry was frowning at the three of them.

"Harry, you know how much better a fighter Professor Vector is than we are," Hermione reminded him. "You'd only slow her down. Besides, if it is you he's after, you're better off getting away and letting someone older handle it."

Harry looked like he really wanted to tell off his friends. It made him all the more annoyed that he couldn't fault her logic. "Fine," he grumbled. "We'll do it."

"Harry, I really am sorry," Hermione repeated. "I never thought we could get this far."

"Yeah, me either," Ron added. "I had no idea what was going on back there with the arithmancy stuff. I don't know if even Bill could've made sense of it."

"It's alright," Harry said halfheartedly. "Let's just stop Snape."

They walked forward into the next chamber.

"What is this place?" Hermione whispered.

It was another torch-lit room, this one considerably dimmer, since there was so much stuff in the way. They were behind the black pieces of a huge chessboard, which stretched from wall to wall. The pieces were life-size and larger, with the pawns being about five-foot-six, and the two kings at least seven feet tall. Far across the board, all the white pieces were glaring at the intruders. There was a large gap behind each end of the board, but the strangest part was the ceiling. The ceiling was completely covered in razor-sharp stalactites, which hung down nearly to the pieces' heads and were constantly waving back and forth with a soft grinding sound like upside-down grass in the wind. There was certainly no room to get through above or around the board, and, to top it all off, there was a heavy feeling of magic in the air.

Professor Vector waved her wand for a moment and then put it away. "The charms in here are very thick," she said. "I believe Professor Flitwick and Professor McGonagall worked together on this obstacle. We're very limited in the magic we can do safely."

"Makes sense if have to play," Ron said. He walked up to the Black King. "Do we, er, have to take your places to get across?" he asked.

The King turned around and nodded, and then all of the King's Side pieces turned and walked off into the gap behind the board.

"Alright, then." Ron squared his shoulders. "Professor Vector, I think you should be the king—"

"No, I think you should be the king, Mr. Weasley," she countered. "It'll be safer for you that way."

Ron should his head. "No, you're the one who needs to go on, right?"

"Yes, but you have to able to finish the game."

"Professor," Hermione said, "What happens when a piece is captured?"

"I don't know, other than that they won't be able to go on. You'll have to be careful about captures, Mr. Weasley. If it's too dangerous, we'll have to turn back."

Ron nodded. "Uh huh, but you still need to be the king, Professor. The game doesn't end in this room. If you're the one going forward, you're the most important piece to protect."

Vector considered protesting again, but she stopped herself. The logic was sound. "Very well. I will take the king's square."

"Good," Ron said. "Harry, you take the bishop's square. I'll be the knight. Hermione—the castle…Let's play."

No sooner were they all in position, than White's d-pawn, a life-sized armoured knight with a sword on his hip, confidently strutted forward two squares.

Ron nodded at the move and then called out, "Pawn to d5!"

An identical black pawn strutted forward two squares, coming nose to nose with the white one. The human players noticed that, unlike normal wizard's chess, the pieces were all silent. Then, White's c-pawn also moved two squares forward.

Ron turned to Hermione when he saw the new position. "Queen's Gambit," he said. "What do you think, accept or decline?"

"Or something else?" Hermione replied. "I don't know; you're the better player."

"I suggest decline," Professor Vector said. "We need to play a defencive line, and that's one of the strongest ones."

"Hmm…good idea," Ron agreed. "Pawn to e6!"

The e-pawn moved forward one square. Then, a little to Ron's and Hermione's surprise, McGonagall's pieces continued playing the Orthodox Line to their Queen's Gambit declined, with one of the White Knights moving to c3. Ron, however, decided to take the game off the Orthodox Line. "Harry, e7," he said. Harry moved one square forward along his diagonal.

White took advantage of the opening, and the white c-pawn stepped forward to make a capture. The human players all gasped as he bent down and grabbed their d-pawn by the wrist and ankle, lifted the statue that must have weighed five hundred pounds high in the air, swung him around for a full revolution, and then threw him over the heads of the other pieces, the stalactites folding out of the way, until he slammed into the back wall of the room behind them and slumped to the floor as if unconscious.

"That…doesn't look good," Hermione squeaked.

"Nothing for it, though," Ron said grimly. "We've got to keep playing."

"Professor?" Hermione said nervously.

Vector thought for a long moment. "Given the seriousness of the circumstances, I suppose I'll allow it. But do be careful. That doesn't look lethal, but it will probably put you in the Hospital Wing should you be unfortunate enough to be captured."

"I'll do my best, ma'am," Ron said determinedly.

They played on. It was a conservative game for a while, mostly deploying pieces, making a few captures on both sides. But then, it Harry's turn to capture a piece.

This was a problem they hadn't thought of before. The White Pawn didn't move as Harry walked up to it, standing on the corner of its square. "Well, move!" he ordered, but the statue remained motionless with its arms folded across its chest.

"Maybe we have to throw it, somehow, like the other pieces do," Hermione suggested.

"But I can't do that!" Harry protested. "Shouldn't the magic take care of it or something?"

"Not on its own," Vector said. "I suspect that's part of the challenge. But a Levitation Charm might work. I think the charms on the room may allow it." She pointed her wand at the White Pawn, and said, "Wingardium Leviosa." With a great effort, she lifted the statue up into the air and tipped it on its side. Sure enough, the waving stalactites folded out of the way, giving her room to move it over the heads of the other pieces. She clutched her wand in both fists as the weight of it took its toll. With her arms shaking and sweat dripping down her forehead, she finally released her spell, and the pawn fell to the floor behind the board with a mighty crash. She slumped in her square, barely keeping her feet. "Phew. Let's hope we don't have to do that too many times."

But a few moves later, Ron had to capture a bishop—a nasty looking creature with a real Cardinal Richelieu look. Standing six feet tall plus the full clerical robes and mitre, it was quite a bit heavier than the pawn. Professor Vector still tried to lift it, as before. She stood on her square, huffing and puffing and clutching her wand tight, but she could only make the Bishop rattle from side to side, lift a couple inches in the air, and then crash back down onto its feet. The statue smirked at them silently.

"I'm sorry. It's too heavy," Vector said. "I don't know how else we can move it."

"Wait!" Hermione jumped in. "Remember the troll? We can lift it if we work together. Everyone cast the spell at once. One…two…three…"

The Bishop got a satisfyingly dismayed look on its face as it lifted into the air. It was hard to lift. Hermione's legs were wobbling from the effort, and Harry started sweating so much his glasses looked like they might slide off his face. Hermione could feel why Professor Vector had so much trouble as she felt the energy drain out of her, but they managed to move the Bishop off the board.

"Blimey, that was tough," Ron exclaimed. "I hope we don't have to do that with the King."

"Yeah, me too," Hermione wheezed.

The game seemed to get even tougher after that. Ron needed a long time to consider his moves. Hermione and Professor Vector offered bits of advice at times, but they could see that Professor McGonagall and Ron were both out of their league. "A transfiguration can only do as much as the caster can visualise," McGonagall had drilled into them. Unfortunately, she could apparently visualise a lot when it came to chess.

"Harry, there's something I've been wondering," Hermione said to pass the time as she watched from her spot on the back row.

He turned around to look at her. "What's that?"

"How did you get past the Devil's Snare?"

At that, to her surprise, Harry actually smiled. "Well, the vines grabbed me," he said. "And I didn't know what they were, but I didn't like the look of it, so I just thought, 'What would Hermione do?'"

Hermione turned bright red. "And…?" she said nervously.

"I used that blue fire spell you're always using."

"The Bluebell Flames?" she gasped, and then smiled herself. "Harry, I didn't know you could do that spell."

"Uh, yeah…me either."

"Oh…" Hermione said. "Well…it's a good thing you got it, then."

"Yeah."

"Yes, indeed, Mr. Potter," Professor Vector said. "When I didn't see you before I reached the Devil's Snare myself, I feared the worst for a moment."

"Hmm…Professor," Hermione said curiously, "Since ordinary fire repels it, Devil's Snare doesn't seem like much of an obstacle to an adult wizard, now that I think about it."

"That's because it wasn't an obstacle. It's there to provide a soft land to anyone trying to enter legitimately. When dried, Devil's Snare makes for excellent matting."

"Oh."

There was a loud crash as Ron directed their other castle to make a capture.

It was a tough game. They lost the Black Queen while managing to capture both of the White Bishops. Hermione was a little annoyed that Ron seemed to be holding her back. She was still stuck on the back row, facing down a White Rook on her diagonal.

Hermione was also starting to really dislike the White Queen. Well over six feet tall and cast in gleaming marble with an elaborate crown and flowing royal robes, Hermione thought she would fit perfectly as the White Witch from The Lion, the Witch, and the Wardrobe. She also looked more and more smug and condescending each time she picked up a pawn and threw it like a rag doll behind the board.

And then, Hermione realised, Ron stood one move away from an easy checkmate, but unfortunately, it was on the wrong tempo. The White Queen noticed this, too and she stepped forward and threw yet another pawn behind Hermione's head, attacking the square where Ron needed to go. Worse yet, Harry was on the same row and was also under attack.

"Uh, Ron…?" her friend said nervously.

"I'm thinking, I'm thinking," the redheaded boy said.

Hermione surveyed the scene. They needed to block the White Queen's attack on Harry and reopen their line of attack to the White King—and be wary of the consequences. This was a tough problem. She wasn't certain she could work that many moves ahead correctly, but one look at Ron's face as it grew more and more grim told her all she needed to know. There were only two choices, and she knew which one he'd make.

"Ron," she said worriedly, "please tell me you're seeing something I'm not."

"No, 'fraid not," he replied.

"What is it?" Harry asked.

"We can win, but…" Hermione started pointing at the pieces. "Our other rook goes to c3, queen takes rook, knight checks king…queen takes knight, bishop checks king, queen to e3, bishop takes queen and checkmates."

Harry could barely follow that, but he saw where Hermione's finger was pointed in the middle of that sequence. "Ron, no, you can't!" he yelled.

But Ron shook his head. "It has to be done. You've got to make some sacrifices in chess."

"But—"

"Do you want to stop Snape or not?"

"Ron—"

"Are you quite sure, Mr. Weasley?" Professor Vector asked, although she was pretty sure she could see the same thing.

"Definitely."

"I'm sorry, but he's right, ma'am," Hermione answered. "The only other choice is rook c3, queen takes rook, bishop checks king, queen takes bishop—" She was pointing at Harry. "—knight checkmates."

"I'll do it!" Harry said at once.

"No!" Ron stopped him. "Harry, I told you the game doesn't end in this room. You keep saying if You-Know-Who…" he gulped. "If he does come back, he'll come after you, and you'll be better off if you're not the one stuck in the Hospital Wing. Just remember, once I check the king, all of White's moves are forced. You'll have to play it out."

"Mr. Weasley, I will try to cast a Cushioning Charm on you," Vector said. "Unfortunately, I can't be sure that it will work. Be sure to brace your head with your arms."

"Thanks, Professor," said Ron uneasily. "Rook to c3!"

The Black Rook moved forward two squares, interposing itself between Harry and the White Queen. But the White Queen just flashed a wicked smile, lifted the Rook up, and threw it behind the lines before gazing down at Harry triumphantly.

Ron took a deep breath. "Well, here I go…"

"Wait!" Hermione yelled. "We win in three moves! Stand aside!"

Nothing happened.

"It was worth a shot."

Vector drew her wand and pointed it at Ron. "Pulvinus," she said, but it wasn't clear if it did anything. "That's the best I can do, Mr. Weasley. I only hope it's enough."

Ron nodded. He was shaking as he stepped two squares forward and one to the left.

The White Queen whirled around as if she'd been offended and angrily strode toward Ron. The boy started screaming, and the other children weren't far behind as she picked him up by one wrist and one ankle, swung him around in a circle, and threw him toward the back wall hard. And then, something happened that they didn't expect: the door back into Professor Vector's chamber slid open, and Ron flew straight through it. There was just enough time for them to hear a thud and an anguished scream of pain before it slammed shut again.

"Ron!" Hermione yelled. She started to move.

"Don't move!" Harry ordered. "We're still playing!" He pulled back to c5 to check the White King. The White Queen, looking distinctly nervous now, moved into Harry's line of fire—her only legal move, if a futile one. Harry moved toward her spot on e3, glaring at her. But then, as he drew his wand, he realised there was a problem. The Queen was even heavier than the Bishop, and there were only three of them now. "D'you think we can lift her?" he asked. A mocking grin appeared on the White Queen's face at his uncertainty.

"We've got to try," Hermione said. "Wingardium Leviosa!"

Two other spells joined her own, and the White Queen started to rattle on her feet, but stayed in place. Her grin grew even wider. Hermione was tiring fast. She was putting so much power into the spell that it felt like she was sprinting flat out, and it wasn't enough. She dropped to one knee, both her hands gripping white-knuckled, and poured out all her anger at what the White Queen had done to Ron into her wand. Distantly, she saw Harry also drop to his knees, and Professor Vector looked like she was wavering. But finally, the White Queen lifted into the air, a look of horror crossing her face. The trio pushed with all their might, and she flew back behind the enemy lines and crashed to the floor.

"Take that!" Hermione choked out.

Harry was panting hard as he pushed himself to his feet and took his place on the e3 square. Praying they wouldn't have to do that again, he faced the White King and yelled, "Checkmate!"

To everyone's immense relief, the White King took his crown from his head and, with a frustrated expression, threw it at Harry's feet with a loud thud, and both the white and black pieces stepped aside to let them pass.

"Yes!" Harry whooped, pumping his fist in the air. "C'mon, let's go."

"Harry, what about Ron?" Hermione called from behind him.

Everyone looked at each other. "He was alive—and conscious. I don't think he landed too hard," Vector said, though she didn't sound too certain. "But just the same, one or both of you should probably go back to help him."

"But what if the other obstacles need all three of us again," protested Harry.

Everyone was silent for another moment.

"How many more are there, Professor?" Hermione said slowly.

"There should be two: Professor Quirrell's and Professor Snape's. I know Professor Quirrell brought in another troll." ("Eep!" said Hermione.) "I don't know what Professor Snape did."

"Quirrell didn't use an anti-dark magic spell?" Harry said in surprise.

"He might have done. If he's any kind of competent—though I'm not convinced of that—he'll have charmed the troll to be resistant to magic. Now, if you two insist on coming along, let me look first, please." Vector walked unsteadily to the door, pushed it open slowly, and peaked inside. Then, she sighed with relief and turned around. "The thief's knocked it out. That should just leave Professor Snape's obstacle…So I'll let you come and take a look. If we can solve it, I'll go on, and you two go back. Understood?"

"Yes, ma'am," both children replied.

"Good. Come quickly."

They walked into the next chamber, still shaking a bit from their ordeal on the chessboard, and were immediately hit with a familiar and very disgusting odour. They held their noses as they tiptoed across the room past a mountain troll that was probably a good fifteen feet tall and even uglier than the one they had met on Halloween, thankfully out cold with a bloody lump on its head.

To their surprise, the door at the far side of the troll's chamber was not closed. Instead, as soon as they walked through the archway, bright blue flames sprang into existence behind them, filling the doorway from floor to ceiling. At the same moment, black flames that seemed to emit darkness and not light appeared in the doorway on the other side of the room.

"We're trapped!" Harry said.

Hermione tentatively reached out a hand toward the blue fire behind them. "Bluebell Flames," she said with relief. "We can get back alright."

"Not forward," Professor Vector said. "That's cursed fire and very hot." It was true—they could feel the heat radiating from it. The only other thing in the room was a table with seven potion bottles and a roll of parchment on it.

"This must be a clue," Hermione said, and she opened the parchment. "Danger lies before you, while safety lies behind…" She read over the poem. "Oh, it's a logic puzzle! Three bottles are nettle wine, three are…poison, and one will protect the drinker to move ahead. Yes, that's brilliant. Snape's always saying how a lot of the greatest wizards haven't got an ounce of logic. This should be easy, then. Just give me a minute…" She started muttering to herself and pointing at the bottles. It wasn't long before she said, "Got it. It's this one." She pointed to the third bottle in the row.

"Are you sure, Hermione," Vector asked.

"Positive."

"No. I don't think so," Harry jumped in.

"What…?" Hermione said in disbelief. He hadn't even seen the riddle. "Why not?"

"It's too easy. Even if most wizards couldn't figure it out, and even if Snape's not the thief, do you really think he'd make it that obvious? It must be a trap."

"Harry, I know you and Snape have had problems, but I still think you're overreacting."

"Actually, Hermione, I have to agree with your friend on this one," Vector said, to her dismay. "That is the sort of thing Professor Snape would do. And besides, there's always the danger that the thief mixed up the bottles."

Hermione deflated. "Then how do we figure out which one is safe?"

"I dunno," Harry said grimly. They were so close, he thought. There had to be a way. "Is there a way we can test them with magic?"

Vector shook her head. "Not with what we have here."

Harry sighed. Was there another way? Had Snape thought about if someone legitimately tried to get in, and the bottles had been mixed up if? Did he even care? He looked around the room for anything that might be useful, but there was nothing. The room was empty.

Wait, no! There was something. With his Seeker skills, he noticed some things fluttering around the blue flames behind them, and several more of them lying motionless on the floor beside the door.

"That's it!" he cried. He ran back to the door and jumped up in the air, as if snatching for a Snitch. Then he seemed to catch something and ran back and dropped his prize on the table: a very dazed gypsy moth. "We can test the potions on these."

Hermione's eyes went wide. She didn't much care for animal testing, but under the circumstances, she could make an exception. "Harry, that's brilliant! We just need a drop of each bottle. Quick, try to catch some more. I'll test the potions."

Professor Vector watched with interest as the two children became like a well-oiled machine. Harry put his Quidditch skills to good use while Hermione picked up the first bottle and tipped it carefully, letting a single drop of potion fall onto the moth. The insect immediately collapsed and lay still.

Hermione frowned, but quickly moved on to the second bottle, since Harry already had another moth ready. She was soon very grateful that he had thought of the idea to test the potions as not only the first three, but the first five bottles she tested, including the one that was supposed to send the drinker forward, turned out to be poison. On the sixth bottle, one of the ones that Hermione had thought was poison, the moth shuddered, but kept beating its wings when the potion was dripped on it. So that wasn't poison. Now, how to test it in the fire?

But Harry already had the answer: "Wingardium Leviosa." The moth beat its wings futilely against the magic. Hermione grimaced as Harry directed it forward through the air until it passed into the cursed flames. But when he pulled it back, the creature was still fluttering and not even singed.

"Well, that's it then," Vector said solemnly. She picked up the potion bottle. "Mr. Potter—Harry, I owe you an apology. I didn't think it was possible, but it seems all these obstacles were not as secure as we thought."

"It's okay," Harry said reluctantly. "Just be sure to stop Snape—or whoever it is."

She nodded. "Hermione, there are not enough points to take away for how reckless you were in coming here tonight." Hermione paled considerably. "And there are not enough points to give back for your displays of brilliance here—that goes for you and your friends. Please pass that along to Ronald for me."

"I will, ma'am," she said, flushing.

"This is where I must take my leave of you. Go back, help your friend, and get all three of yourselves out of here. Them find Professor McGonagall, tell her whatever you have to to make her listen, and owl Professor Dumbledore."

Harry nodded in understanding. Hermione's lip trembled, and she suddenly dashed at her favourite teacher and threw her arms around her. "Oh, Professor, please be careful," she sobbed.

"Always…Now, go!"

Septima Vector watched as the two children turned and passed through the blue flames with a silent prayer that they would make it back alright. Then, she swallowed the protective potion and felt its icy tendrils flowing through her body. She took a deep breath, drew her wand, and stepped forward through the cursed fire.

A/N: Gules, Or, Vert, Azure, Purpure, and Argent: from the Old French for "red", "gold", "green", "blue", "purple", and "silver", as used in heraldry.

Pulvinus: based on the Latin for "cushion".

In case you were wondering, Alice's secret number was 11,869, and Bob's was 2,574.