Summary of the previous chapter:

Harry and Tom wonder if the Death Curse has something to do with Tom ending up in Harry's head and decide to do some research on it. Harry, having a blonde moment, asks Lockhart for more information after a lesson, which leads to another clash between them. It fires rumours that Harry threatened Lockhart with the Death Curse, causing Professor Snape some concern when Lockhart is found petrified in his office.

Albus decides that Severus should take over the DADA class after the Easter holidays, while he lures Slughorn out of retirement until the summer to teach Potions. He also wants to send all pupils home for Easter this year, and do so a week early. Until then, strong security measures are put in place.

A/N: I really owe a world of gratitude to my wonderful beta, Dreamthrower! She not only corrects the many mistakes in my writing, but also pays attention to consistency problems which keep arising due to the fact that I'm mentally in book four, which often makes me forget that some developments have not taken place yet when editing older chapters. Thankfully, she usually notices and spares you the confusion. :)

Solving the Riddle

After the petrification of Lockhart, the school was in uproar. Not only had he still an astonishing number of fans, but it was also widely known that he was not Muggleborn. Even students who had felt safe before were now panicking. Attacking a cat, a first or a second year was one thing. But attacking the Defence Against the Dark Arts teacher? It made the heir seem even more dangerous, especially since the teacher had been unable to defend himself against his mysterious attacker. Few students accepted the headmaster's assurances that no death threats had been made against Lockhart and that the student who was rumoured to have uttered them had an alibi. Most clung to the belief that Harry was the heir. Tom was convinced that they only did so because knowing who not to cross made the danger seem more controllable.

Socialising under the new safety measures got very difficult for everybody. Teachers or prefects led students everywhere, even bathroom breaks had to be scheduled. In the afternoon, the only places to be for the students were their common rooms or the library. If they chose to meet there, which Harry, Hermione and Neville did, they had to remain until a prefect took them back to their respective houses in groups.

It was far from ideal and everybody was getting very upset with these restrictions of free-time activities. Quidditch matches had been cancelled, too, as the logistics of getting everybody down to the pitch and back were just too much. On a brighter side, they didn't have any more DADA lessons before the holidays. Students were supposed to self-study during these hours, and older prefects were exempt from their own classes in a rotating schedule to chaperone the younger years.

Harry was being treated like a pariah by the majority of students. Hermione and Neville didn't tire of repeating that he had nothing to do with Lockhart and that he had been with them at the time of the attack. It helped very little.

"We have to make another effort to find the culprit," said a very determined Hermione to her friends when they sat at their usual library table two days after Lockhart's petrification. "This madness can't go on!"

"But we already tried and came up with nothing," replied Neville frustratedly.

Hermione was adamant. "Let's start by recapitulating everything we found out so far. I'm sure we've overlooked something." She pulled out a piece of parchment and began to make a list.

What we know for sure:
- Three people, a cat and a ghost have been petrified, cause unknown.
- Mrs. Norris was found hung by her tail above a huge puddle.
- Colin Creevey was found looking through his camera, probably trying to shoot a picture of his attacker.
- Justin and Headless Nick were found in the middle of a corridor during lessons
- Lockhart was petrified while posing in front of a mirror.

"You have to admit the girl is methodical," commented Tom, who was looking at Hermione's growing list through Harry's eyes. "And undeterred. I really like that about her."

A wizard or witch is involved as there was writing on the wall, unless it was a house-elf acting on orders.

House-elves have huge eyes, are probably capable of petrifying and killing and can appear and disappear at random. But foreign house-elves haven't been able to pop into the castle since the hospital wing episode.

Ararog wasn't the monster but claims that a monster exists.

"I guess that rules out house-elves as well," Hermione mused, critically eying what she had written down. "Though it would make sense in a way: House-elves clean. They probably regularly kill spiders and their nest, so they're probably monsters in their eyes. However, he spoke of ONE monster, and the castle has many house-elves."

"If only he would tell us what it is!" said Neville. "Maybe we should try and talk to him?"

"And go into the Forbidden Forest where a hundred other Acromatulas live?" Tom asked, aghast, though his objection remained unheard.

"If Aragog doesn't speak to Hagrid, he certainly won't speak to us," decided Harry, and Hermione picked up writing again.

There's no known beast that petrifies people.
No one was petrified in 1945.
Myrtle, the only victim, was killed on sight.

"Oh my god," exclaimed Hermione, halting her quill all of a sudden and staring at the parchment with widening eyes. "We've been looking at things from the wrong angle when we searched for the monster!"

"How so?" Neville looked at her list to see what had inspired her claim.

"We searched for a beast that can petrify people! But nobody was petrified in 1943 – Myrtle was killed! So the question should have been: What creature has huge, yellow eyes and kills at a glance?"

"But none of the victims this time around was killed," Harry objected. "Only Myrtle died from meeting its gaze. Do you think we're dealing with two different monsters?"

"Of course not! Guys – don't you see? Of all the victims, only Myrtle met the monster's gaze! None of the petrified victims looked whatever attacked them straight in the eye!" Hermione was really getting excited now and pointed to her list. "Mrs. Norris most likely saw its reflection in a puddle. Lockhart met its gaze in a mirror. Colin saw it through the lens of his camera!"

"And Justin?" asked Neville sceptically.

That gave her a moment's pause. "Through Nearly-Headless Nick!" she then declared triumphantly. "Ghosts are see-through!"

"She's right!" Tom exclaimed, stunned. "why didn't we see it before? It seems so obvious now!"

Harry got caught up in their excitement as well. "That's it – you solved the riddle!" he exclaimed enthusiastically, which promptly earned him a sharp rebuke by Madame Pince. "It has to be like you said," he repeated with a lowered voice, but undiminished exhilaration. "We need to repeat our search, only this time for a beast whose gaze can kill!"

"Actually, there's no need…" gulped Neville, blanching. "I know a beast that can kill a wizard with a glance: A basilisk!"

"Isn't that a type of snake?"

Neville nodded. "A very big, venomous and very deadly snake."

Hermione jumped out of her seat in excitement and disappeared down one of the library aisles, presumably to get a book on basilisks.

"Harry! The snake we've been hearing!" Tom said suddenly with a dread-filled voice. "If the monster is a snake, the heir has to be a Parselmouth! It means Riddle must have been the heir! It was he who set the basilisk on Myrtle, it has to be!"

Harry frowned. "But that's impossible. Riddle is you. You'd never do something like that."

"He's not me! You said it yourself! And how great is the chance that there was another Parselmouth at the school at the same time as him?"

"About as big as the chance of there being another one at the school right now, because I'm not the one commanding a basilisk to attack students, and neither are you. So obviously, the ability to speak Parsel is much more common than people believe. It's no wonder though that people prefer to keep it a secret, given the prejudice they face. Besides, Riddle was a half-blood himself. Why would he want to kill Muggleborns?"

"I don't know why! It doesn't make any sense for anybody to attack fellow students, and yet someone is doing it! Why do you refuse to accept what's right in front of you? Riddle IS an heir of Slytherin. He went to school here in 1943. He was a Parselmouth! The attacks stopped after he blamed Hagrid! Everything points to him being the culprit!"

Harry couldn't exactly refute the evidence, but the idea didn't sit well with him. Tom wasn't evil, so how could his past self possibly have been a cold-blooded murderer?

"Maybe Myrtle was a tragic accident," he ventured. There, that was a possibility, right? "The basilisk happened to cross her gaze, and Riddle couldn't prevent it from happening. He didn't mean to kill her – that's why he never said anything. And now another Parselmouth – can't be Riddle, given that he's dead – found the basilisk and is using it to create havoc at Hogwarts and cause trouble for me. That makes it highly likely that the one who calls himself Slytherin's heir is a sympathiser of the Dark Lord."

"Harry!" Hermione waved her hand in front of his eyes, trying to catch his attention as he had obviously spaced out again.

"Sorry. Yes?"

"I found the book!" she said, and triumphantly pointed at the picture showing a gigantic snake. "Look!" Tom didn't say anything more, but Harry could feel that he didn't agree with Harry's conclusion.

"The monster is a basilisk for sure. Listen: The basilisk is the king of snakes and may live for hundreds of years. Aside from its deadly and venomous fangs, it has a murdering stare that kills everyone who looks it in the eye instantly. Spiders flee from it as it is their greatest enemy, but the crowing of a rooster will kill it."

"Hagrid complained that something killed his roosters," Harry remembered.

Hermione looked at them, beaming. "It all fits! It's just missing the information that apparently, a basilisk's gaze doesn't kill but only petrifies if one only sees its reflection."

"She might be a know-it-all at times, but, Harry, her deductive skills are brilliant!" Tom said, much impressed. Harry had to agree, but he played devil's advocate, just to make sure.

"Why doesn't it say so in the book?"

"Well, that one's easy to explain!" Neville snorted. "If you happen upon a basilisk in the jungles of Borneo or wherever, meeting its gaze in the reflection of a river while drinking when it comes up from behind doesn't make a difference. You'll be eaten, and thus not be able to tell anyone of your discovery. The circumstance of having a basilisk in a castle is rather unusual."

"But look at the picture!" Harry tapped his finger on it. "This thing is gigantic! Surely someone would have noticed a huge snake crawling about? Unless it can turn itself invisible, but I'd find that hard to believe."

Hermione had to think about this for a moment. "The pipes in the walls," she finally suggested. "It might be going through the pipes. I know normal pipes are much too small, but maybe wizarding pipes are, I don't know – flexible? Like our trunks, bigger on the inside than the outside?"

"And why should pipes be this flexible? In case you need to flush a person through the toilet?"

"Well, actually …" offered Neville, looking at them pensively, "flushing people through pipes is a thing at the Ministry …"

"What?" Hermione and Harry started at him in utter bewilderment.

"It's how many employees of the Ministry of Magic get to work. They flush themselves through public toilets and emerge from the fireplace in the Ministry atrium. My grandma told me. It's how my parents got to work when they were still apprentice Aurors. Usually, it's only low level employees who don't have a Floo clearance for the Ministry who use the public entrance. And of course, all those who live in the Muggle world and don't even have a fireplace at home."

"You've got to be kidding!" exclaimed Tom, aghast.

"Ew!" Harry was disgusted as well.

"It doesn't make any sense," protested Hermione. "Why would the sewer pipes of a toilet be connected to a fireplace? The Ministry would be a sewer! I don't want to even think about what employees would be wading through to get to their offices."

"No, they don't. When they arrive in the fireplace, they're all dry."

Hermione remained sceptical. "I really don't see how that could possibly work."

Neville shrugged. "I don't really know, but it does."

"What if a Muggle uses the toilet?" Harry wondered.

"Well, I think a Muggle usually wouldn't stand in it while flushing, would he?"

"Ehm, no. Definitely not."

"There you have it. It's probably just a regular toilet then. I know wizards have magical tokens to operate the toilets. I guess they lock and unlock the doors again after a while."

"He has a point there," Hermione admitted, after having thought it over. "I think no Muggle ever uses public restrooms, especially not the old ones, unless it's an emergency. Probably not even then."

"It's still disgusting! Why a toilet?"

"I suppose they needed a place people could enter and not be observed," Neville suggested. "A place no Muggle would notice if you came out again or not."

"So, okay – flexible pipes might be a thing," Harry cautiously allowed. "But the basilisk still has to come out somewhere. Are we supposed to look for broken pipes? Or does he come up the drains?"

This thought filled them all with horror. Imagine sitting on the loo and …

"Nobody was killed or petrified while doing his business," Tom pointed out. "If at all, it probably comes through clear water pipes."

"That might be how he got into Myrtle's bathroom!" Hermione exclaimed excitedly. "Or why the floor was flooded!"

"She's right, Harry," Neville agreed. "Almost everything fits."

"What do we do now?" asked Harry, feeling a bit overwhelmed. "How do we find it?"

"YOU don't find anything!" Tom told him sternly. "Didn't you listen – the thing kills on sight if you happen to cross its gaze unreflected! YOU will go to Professor Snape and tell him of your findings, and then let the adults deal with it for once!"

*'*'*'*'*'*'*'*

That's what they ended up doing. Immediately. Since Madam Pince wouldn't let them leave the library without escort, no matter how urgent they claimed their plight was, the trio disappeared between the rows of shelves and slipped under Harry's invisibility cloak, which he had taken to carrying around. Strangely, it didn't take up much room in his backpack, but was large enough to fit all three of them under it. Even so, Harry didn't have to worry about stepping on the seams when using it by himself, the cloak obviously adjusted somewhat to the needs of the wearer. The fact that they had to tread carefully so their feet wouldn't be seen suggested that it had its limitations though – they probably wouldn't be able to hide an elephant beneath it.

Professor Snape's face darkened when he saw who entered his office after being asked in. Before he could berate the three second years for roaming the corridors all by themselves, Hermione exclaimed: "We know what the monster is, and we thought you should know immediately!"

Severus watched in disbelief as the infernal trio planted themselves in front of his desk, moving papers and parchment aside to make room for a heavy tome they had obviously stolen from the library. Miss Granger opened it at a page marked by a sheet of parchment stuck between pages – a list of some kind – and turned it around for him to see.

"It's a basilisk, Sir," said Miss Granger in the overly didactic tone that so often grated on his nerves, pointing to the page in question. With an air of importance, she then launched into a long and elaborate explanation, which the boys kept supporting with repetitive approving nods, making them look like bobbleheads.

"Pipes?" Severus asked, mystified, when she got to that point. "Self-expanding? Why would the plumbers have installed expanding water pipes in the castle?" To be totally honest, he had never understood why they had put water toilets in a magical castle in the first place. All the waste was transported into the Hogwash, the magical clarification plant that lay hidden in a valley between the hills, under a magical dome to prevent smells. Hagrid grew fertilizer from it, but that hardly justified the hassle of installing indoor plumbing into a century old building. It'd be just as effective to do one's business in chamber pots and simply vanish the contents right away, as wizards had done for centuries. Though admittedly, not all students were good at vanishing spells, so maybe it was better to trust water flushing.

"Well, we thought like the toilet entrance to the Ministry …"

Severus shook his head at the absurdity of the thought. Really, they needed more magical theory in their curriculum. The children really seemed to believe wizardry was just stark naked magic. "No one is going through sewer pipes, certainly not the Ministry employees! The toilets are charmed to act as fixed and permanent Portkeys into the Ministry's Floo. I assure you that flushing yourself down a toilet at Hogwarts will not get you to the Ministry. I advice to refrain from trying."

"It could still be going through the pipes, though," the girl insisted. "It doesn't say anywhere that basilisks can't swim."

"And how would it fit into the pipes given that they are not 'self-expanding'?"

"The basilisk might be small," Miss Granger argued. "The information on their size is a bit contradictory. While most sources, including Newt Scamander who's certainly an expert on magical creatures, claims that they can grow to gigantic proportions, Pliny the Elder says that basilisks don't get very large: only the length of 'twelve fingers'."

Severus pondered the information brought before him, looked at the parchment that summed up all the evidence they had collected, and the book presented as supporting source for the basilisk theory. It had, he had to admit, some merit.

"How do you know about the 'huge yellow eyes'?" he inquired.

"Myrtle told us. The ghost of the girl who was killed in 1943."

Severus frowned. "I have never met a ghost of that name in Hogwarts."

"No, you wouldn't have, Sir. Unless you ever had a reason to be in the girls' loo on the second floor."

The defective bathroom that aggravated Filch so much because it often flooded for no apparent reason. It seemed he now knew the cause.

"You are raising good arguments," he conceded. "I'll have to run this by the headmaster. You three stay put."

Harry, Neville and Hermione watched as the professor got up and walked over to the fireplace. From the tin on the mantle, he took some powder and threw it into the flames, which turned them green. Bending closer, Severus called out for the Headmaster. "Albus, we have new information about the monster and I have visitors in my office. Could you please come through?"

*'*'*'*'*'*

It didn't take long for the headmaster to arrive. Harry and Hermione watched in fascination as he stepped out of the fireplace. It seemed physically impossible, as the fireplace was much smaller than the headmaster was tall. And Professor Snape had found it laughable that they had believed pipes to be self-expanding!

The headmaster was surprised to find the three of them in the Potion Master's office and voiced his hopes that they hadn't got into mischief. Professor Snape allowed Hermione to explain once more what they thought to have discovered, and the headmaster listened with rapt attention.

He was visibly thrilled by their findings and didn't think it absurd, not even their idea that the basilisk was travelling through the pipes.

"But Albus – it can't possibly be going through the pipes, they're too small!" Professor Snape objected.

"Oh, I'm sure it's not going through the pipes," declared the headmaster. "It's going through the walls! Many walls of the castle are double walls that have some space between them – wizarding space." The headmaster looked at Harry. "I heard you are the owner of a magically expanding school trunk? Then you're familiar with the concept. Hogwarts is known to expand rooms, shrink them or add new rooms between existing ones if they are needed. The helpful shortcuts that allow staff to get from one point to another much quicker than students, are in truth passages in the wizarding space between walls."

"That's not how wizarding space works," objected the Potions Professor. "Sure, it can be magically expanded, but it still needs a wizard to cast a charm and define the parameters. It's not fluctuating, adapting to anyone who walks or slithers through it!"

"Unless basilisks have the magical ability to warp wizarding space," the headmaster replied, unfazed. "We know too little of them to outright dismiss the possibility."

It still seemed far-fetched in Severus' opinion, but he had to admit that everything else seemed to fit. "So we're running the risk of meeting a basilisk when using the shortcuts?" he asked, deeply unsettled by the thought. "We have to inform all staff that we can't use them until the creature is found and killed!"

"How would you kill it?" asked Harry, "when you can't risk looking at it?"

"We can always set roosters loose all over the castle at dawn," replied his Head of House, "and have chicken soup in the evening."

"Ah, but how many roosters would we need, how would we get them and where would we place them?" asked the headmaster, stroking his beard. "We don't even know where the Chamber is. So how do we make sure that the roosters' cries reach the basilisk? We've had roosters at Hogwarts for many years, and yet, if what you're suggesting is true, this basilisk is still alive. And if we were to go through with the plan, how would we know whether it was successful? No, I think we're in need of an expert opinion. Someone who can tell us how to handle this situation."

"There are experts on basilisks?" Tom wondered. What a curious profession. Especially since the creatures seemed to be extremely rare.

"I happen to know someone who can probably help us there: My good old friend Newt Scamander. You have all read his book 'Fantastic Beasts and Where to Find Them' in your first year. I shall send him an owl right away!" The headmaster looked at them over the rim of his glasses, a very pleased smile on his face. "Well done, you three! Really, well done!"

A/N: It's not clear if the toilet entrance to the Ministry existed before Voldemort took it over, but I think it'd make sense. Allowing every employee to Floo straight into the Ministry would pose a security risk. It was probably a privilege for trusted senior employees, and under Voldemort, many of those formerly high-ranking officials saw their privileges revoked, meaning they had to use the public loo like low-level employees and wizards without Floo-access again.

You can find an essay on the Hogwarts Hogwash, the water purification plant, here on Ao3. I just couldn't resist using the term. Hogwash – isn't that brilliant?

Hogwarts Waste Managment Facility by Librasmile (Tenthsun)