Saturday morning dawned crisp and clear on an unexpected phenomenon. John Watson and Poppy Pomfrey arguing in the hospital wing.

"Look, it's fine!" John said, walking up and down the length of the room.

"I'm sorry, John, but I can't clear you," Madam Pomfrey said, genuine regret on her face. "I've already sent an owl to Healer Gillysmythe and he agrees."

"I've been walking around on it for three days without the cane. No pain, no cramps, no weakness. Healer Gillysmythe always said it was a mental thing."

"Yes, and that's what makes it so imperative that we continue to observe you. What caused this sudden turnaround?"

John stopped his energetic pacing and faced her, squaring his shoulders. "Why is that important?"

"Because we need to know how it affected you. If it's something that can be used to help other psychosomatic sufferers. If it's permanent. If you'll wake up tomorrow and your leg will be worse than ever."

Poppy had mirrored his aggressive stance, shoulders back, arms folded, chin down. John knew a lost cause when he saw one. Madam Pomfrey with her heels dug in was more frightening than a nest of acromantulas. He toyed momentarily with the idea of telling her exactly what caused the limp to disappear. He thought it likely she would keep his secret, and he technically would be serving time for it later that day. But the tale implicated others. He settled for a non-specific version of the story.

"I had a foot race up the stairs from the ground floor to the Gryffindor dormitory. Didn't miss my cane till I was at the top."

"What inspired you to sprint up seven flights of stairs? When I saw you Monday, coming down two flights to get here was a challenge. That's only five days past –"

"But the point is that it's gone now. Isn't that what you and Gillysmythe have been working for?'"

"Yes, and when enough time has passed that we're certain it won't come back, you'll be free to do whatever you wish. But I can't give you permission to spend hours on end on a broomstick dozens of feet off the ground. Quidditch is dangerous enough without the possibility of your leg giving out and you plummeting to your death. I'm a good healer, John, but I can't heal that."

It would be easier if he didn't know her so well. Poppy Pomfrey cared about keeping the students of Hogwarts safe. Nothing else in all the world could possibly matter as much as that – particularly not Quidditch. He'd been treated to a half hour lecture on the dangers of the sport when Snape had banned it last year. Madam Pomfrey had almost sent him a bottle of mead.

He sighed and leaned forward "Look, I've only got this one chance left to be on the Quidditch team. I've tried out every year I could and never made it. There's no guarantee I'll make it this time. But I want to try."

She wilted a little, holding his gaze. He could see the hesitation in her eyes.

"I'm sorry, John. No."

Sherlock was waiting outside the door, leaning against the wall and tapping his foot. "Well?"

"Let's go," John growled, taking the first flight of stairs two at a time.

Sherlock caught up by the sixth step. "The chances of you being on the team were never high, anyway. Ginny Weasley is captain, right? That means that every member of Gryffindor house who owns a broom will be out on the pitch. And you've tried out before, haven't you?"

"Yes, but the first time the Keeper position was available, I was a 4th year, and all the other people who tried out were older –" John let his voice trail off. "I didn't say she'd said no."

Sherlock merely looked at him. John had to admit it wasn't too difficult of a deduction. They reached the second floor landing and turned the corner to the next set of stairs when Sherlock spoke again.

"The probability is that you wouldn't have been selected this year, either. Given your lack of playing experience and the fact you haven't been on a broom in over five months, there are bound to be at least three players who would outfly you, probably more considering the Gryffindor proclivity for Quidditch. Madam Pomfrey probably saved you from a great embarrassment."

John stopped and turned to face Sherlock. The Ravenclaw stopped as well, one foot on the next step down, and looked at him expectantly. John didn't speak.

"What?" Sherlock asked. "I was just stating facts."

"Well, stop."

"It's nothing about you," Sherlock said, his tone more scornful than apologetic. "It's just probabilities. Why are people always so quick to think everything is about them?"

John started down the stairs again, giving the sarcasm free rein. "Because we're all incredibly self-centered. But thank goodness you're above all that."

He glanced over to see Sherlock's reaction, but the boy's attention had been caught by a student standing near the foot of the stairs. John's brow furrowed as he squinted down at the boy. His robe was trimmed with Slytherin green, and he had a somehow reptilian stance. Perhaps it was the way his head seemed to slide from side to side rather than turn, or the beady look of his black eyes, but John instinctively disliked him.

"Sherlock," the boy said, his Irish accent oddly threatening in the soft tones.

"Jim," Sherlock said, giving the widest fake smile John had ever seen. "I was wondering when I'd see you."

"Miss me that much?"

"Don't flatter yourself."

"Excuse me," John interrupted, disliking the subtle animosity. "Who are you?"

The boy stuck out his hand with a slight bow. "Jim Moriarty, hi."

"John Watson."

Jim let go of his hand and jerked his head at Sherlock. "Why are you with this one? Some sort of punishment?"

"He's my friend," Sherlock said, bristling.

"And we were just heading to the Quidditch pitch, so –" John tried to edge toward the door.

"Friend?" Moriarty grinned rather nastily at John and turned to Sherlock. "I didn't think you bothered yourself with ordinary people."

"Don't you have some sort of plotting to do?" Sherlock asked, rolling his eyes. "Some prank to pull, or argument to start, or anything?"

"Bored with me already? That won't do, Sherlock." The Slytherin gave another slow, skin-crawling grin and backed away, hands in his pockets. "I'll be seeing you."

John and Sherlock stood stock still until he entered the Great Hall, apparently intent on catching the tail end of breakfast.

"Who the bloody hell was that?" John asked, his voice strangely quiet to his ears.

"Jim Moriarty," Sherlock said, folding his arms across his chest in an almost defiant way. "We've run into each other a few times. He likes to cause trouble where he can – mostly arranges other people's dirty work. If you have someone you want to get even with or generally make miserable, you talk to Jim Moriarty and he either does it or arranges for it to be done. I've managed to find proof against a couple of his goons in the past, but never Moriarty himself. He's clever that way."

There was no mistaking the note of begrudging respect in Sherlock's voice. He was impressed with the Slytherin, and John thought he knew why. It would take an extremely clever boy to leave no clues that Sherlock could find, and Sherlock would have no choice but to find that fascinating. John wasn't sure what was more disconcerting: the fact that Sherlock seemed somehow delighted in Moriarty's talents, or the idea that this boy had been operating in the school last year and the DA had known nothing about him.

"Well," John said as the silence lingered. Sherlock was still looking at the entrance to the Great Hall. "Shall we head down to the pitch?"

"Why? You're not going to try out."

"Because one of my best friends is still going to be flying and I want to be there to see it."

Sherlock looked at him as if he was speaking a foreign language. "Sentiment," he said, the word almost a question.

John sighed. "You could call it that."

He was somewhat surprised when Sherlock nodded and fell in step with him. They headed across the lawn and down to the pitch, where figures where already darting through the air. John counted at least 12, which meant that Sherlock was right – most of Gryffindor house would be out. He wouldn't go so far as to say that Sherlock was also right about him ending up embarrassing himself, but it was, all in all, perhaps best. The knowledge irritated him more than it soothed him.

They were passing the greenhouses when another student came running out to meet them. John had only processed the fact that his robes were trimmed in blue when Sherlock recognized him.

"Victor, surprised to see you out this early."

The boy brushed back his long brown hair and grinned. "Had some research to do. I wanted to tell you that the potions matured on that sample you gave me. I wrote out some findings, but you won't be happy."

Sherlock took the thin roll of parchment and opened it, brow furrowing. "Are you sure?" he asked, looking up.

"Positive. I even duplicated the substance and ran two tests to check," Victor said. "It doesn't match up to any creature in Wizarding Records. I went through the appendices of Fantastic Beasts three times last night. The best I can figure is that it's some sort of hybrid, probably something illegal. Hagrid, do you think?"

Sherlock nodded.

"Now wait just a minute," John protested. "Just because it might be a hybrid beast doesn't necessarily mean that it's of Hagrid's making. We have the entire Forbidden Forest full of creatures that we don't know about right at our doorstep."

Both Ravenclaws stared at him. John straightened his shoulders and endured the scrutiny. "It's true, you know. Besides, Hagrid has been busy helping rebuild the castle all summer. I sincerely doubt he had time to do any interspecies experiments."

Sherlock raised an eyebrow. "Very good, John. Fair reasoning. Of course, you're leaving out the fact that Hagrid has access to a great number of magical creatures, and it is quite possible for them to… create hybrids without his encouragement. Regardless, the fact remains that Hagrid has a reputation for trying odd experiments, and rather welcomes ."

"Let us all remember the Blast-Ended Skrewts," Victor interjected.

"Probability doesn't equal guilt," John maintained. "I think if Hagrid knew anything about this creature, it would have been taken care of by now. McGonagall wouldn't let him keep something that dangerous."

Victor looked at Sherlock and shook his head. "Skrewts."

John and Sherlock were still bickering about Hagrid's supposed guilt or innocence when they arrived at the pitch. The seeker tryouts were just ending, and the chasers were queuing up when they took their seats. John located Lindsay near the end of the line and gave a wave, which she returned distractedly.

"Look, all I'm saying is that we're making a huge assumption that is even is a hybrid, much less that it's one that originated here, and that Hagrid was involved," John said, watching the second batch of chasers dodging down the pitch.

"An assumption, yes, but a logical one. It fits. The reasoning is sound," Sherlock replied evenly. He'd taken a piece of parchment and quill from his robes and appeared to be working some sort of equation, taking information from the parchment Victor had given him. "This genetic code here –it's very similar to another one I've studied…"

John waited for him to continue, keeping one eye on the pitch. These chasers were pathetic. Lindsay was standing with her arms crossed, body tense. He couldn't see her expression, but, having played three-a-side Quidditch with her in the Room of Requirement last year, he could guess as to its meaning. He could almost hear the sighs of relief around the pitch with the group landed and the next set took to the air.

"Lacewing flies!" Sherlock exclaimed, holding up the parchment. "See? These markers – they are almost identical."

"So… a fly killed that house elf?" John asked, hoping the question was as insane as it sounded to him.

"No, no, of course not," Sherlock snapped. "It's just this one part here. The lacewing fly is an ingredient in polyjuice potion because it has the ability to change its skin pigmentation to reflect its surroundings. A bit like the disillusionment charm. Whatever this creature is, it has a similar ability."

"Do we have live lacewing flies on Hogwarts grounds?"

Sherlock considered for a moment. "I've never heard of them or seen them, but that doesn't mean they aren't around."

"But it does lower the likelihood that Hagrid is to blame, doesn't it?"

"What it does is convince me that whatever this is, it's not a typical hybrid. No creature that large just 'happened' to be crossed with a fly, and Hagrid isn't the type to do laboratory-style crosses. Which means –"

"Shush," John said suddenly. "Lindsay's going up."

The last group of six chasers mounted their brooms and took off to opposite sides of the center line. Ginny tossed the quaffle up, and Lindsay caught it on the tips of her fingers. She tucked it under her arm and took off, guiding her Cleansweep 10 down the pitch to the goalposts. She dodged under the reaching arm of another chaser, then swerved to the left to avoid another. One of the chasers on her side raised a hand as two from the other side came at her, but Lindsay did a hairpin turn and climb that left her above and behind them. She fed the ball off to the other girl, but a boy from the other side intercepted it right at the girl's fingers. Even from the stands, John could see Lindsay's body tighten in reaction. In the time it took him to groan in frustration, Lindsay had caught up to the boy and had swiped the quaffle almost out of his grasp. He just barely maintained his grip and spun around to get out of her reach.

"Oh, he's got a Nimbus. The turning radius on that thing is incredible," John said.

Sherlock looked at him blankly.

"Just… go back to your genetic markers and whoosy-what's-its. I'll let you know when the trials are over."

Sherlock bent his head over the paper almost docilely. John had to laugh as he returned his attention to the pitch. The boy was almost to the goalposts, but Lindsay was closing the gap between them faster than a Cleansweep 10 should be able to when matched against a Nimbus. He raised his arm to throw from outside the keeper's range, and Lindsay tapped the quaffle out of his hand. It dropped neatly into her own and she had spun around and was back down the pitch before the boy had lowered his arm. She scored a goal before anyone managed to catch up to her.

"She'll get in for sure," John said jubilantly, turning to Sherlock.

The Ravenclaw didn't look up from his parchment.

They didn't manage to catch up with Lindsay till lunch. Sherlock had insisted on leaving early to look up some obscure book in the library, and John, after a moment's hesitation, had followed. He hadn't been able to follow most of the information Sherlock shared from the book he'd found, but gathered it basically only confirmed what they already knew – there was no magical being that fit the criteria for the monster they sought.

When they entered the Great Hall, Sherlock surprised John by following him to the Gryffindor table and seating himself as though this were an everyday occurrence.

"Shouldn't you eat with the Ravenclaws?"

Sherlock shrugged. "It won't hurt the house elves to move one plate, and I don't like to eat with my classmates."

Lindsay dropped onto the bench across from them, a tired but ecstatic smile on her face.

"Well done!" John said warmly. "You were the best out there."

Lindsay's face tinged pink. "Thank you. Ginny said I'll have to learn a bit about sharing the quaffle, but she's excited to have me onboard."

"Not if sharing means the other people are going to give up possession," John objected. "But listen, we found out some stuff about the creature."

Sherlock looked up from the orange he was slowly peeling. "I'm not sure we can call it a creature anymore," he said, smiling at the confusion on their faces.

"Well, then, what was it? A person?" Lindsay asked.

"No."

"Plant?"

"No."

"Why don't you just tell us instead of going through this pointless guessing game?" John asked. "You know we'll never get it."

Sherlock put the orange down, laced his fingers under his chin and leaned closer to both of them. "It's not a magical being at all."

Lindsay and John looked at each other, but neither face held any answers.

"Say again?" John asked.

"Think about it," Sherlock said. "We've looked up every magical creature known to Wizardkind. We've actually had tests run on its DNA, and nothing. Not a single mention anywhere of anything that resembles this thing. When you've eliminated the impossible, whatever remains, however improbable, must be true."

"So, what, it's a… a Muggle beast?" John asked.

"A wizard who is an animagus?" Lindsay suggested.

"No, neither of those explain the skin pigmentation of the sample. And the bit I plucked out of the window last night was a completely clear hair. There are no creatures with clear fur. So, what does that leave us?" He waited, but both of them were looking at him. The right side of his mouth tipped up in a smirk. "Alien."