John had always thought that his life with Sherlock had prepared him for anything, but he was wrong. He was not prepared for Hogwarts School of Witchcraft and Wizardry. He wasn't even prepared for the name, John reflected wryly, wincing and rubbing his temples for the third time in as many minutes as he trailed down the corridor after Sherlock. Witchcraft and wizardry. By rights it should not have been possible for his flatmate to get any weirder. But of course he had. Of course Sherlock would be a wizard. Of course he'd have spent his childhood in this madhouse, conversing with the portraits and learning to levitate furniture and make giant fires implode into being. The bullet holes in the wall and bloody specimens in the fridge were probably just some sort of subconscious response to the dilemma of no magic. Bored, indeed.

And of course John Watson would be trailing along behind him without question, tracking an escaped murderer through this labyrinth of a school in some bloody mental caricature of their version of 'normal'.

Shocking as his first glimpse of Dumbledore's office had been, it was becoming painfully clear that it had been a rather low-key prelude to the rest of the school. The corridors themselves were ordinary enough, aside from the dozens of gossiping portraits and suits of armor that snapped to attention as they hurried past, but when the first staircase they stepped onto started moving, John clutched at the railing, suddenly queasy. Sherlock glanced back at him.

"All right?"

"Fine," John exhaled, casting a distrustful glance behind him as the staircase started rotating again. Sherlock, fortunately, seemed to have expected it; from what John could tell he knew the castle better than the backstreets of London. Which was very well indeed.

Twice the detective ducked into stone passages hidden behind tapestries, and once, as they passed a blank section of wall, Sherlock let out a hiss between his teeth and dragged John into a doorway that he was quite certain hadn't been there a split second before.

"What're you…"

"Peeves," said Sherlock, by way of explanation, and they were off again, dashing down deserted corridors full of bearskin rugs and old-fashioned candle sconces. By the time they pulled up, abruptly, outside one of the (to judge from the worn stone) more frequented passages of the castle, John found himself grinning, something like the usual mad joy of the chase thrumming through his veins. Though Sherlock's face was inscrutable as ever, John could tell he felt the same way.

Sherlock glanced out the window at the heavy sky.

"Nearly half past five, wouldn't you say?"

"Why guess? You never leave your mobile…"

"Won't work here. Too much magical interference…another point of study," Sherlock muttered to himself, tapping his wand against his leg in thought. Then he came back to the present. "Nearly supper. Might still be a late class or two getting out. Here—"

And he pulled the wad of silvery material from his robes, throwing it around himself and John.

"Hey—!"

"Shh!" hissed Sherlock, pulling John back against the wall.

Seconds later an invisible bell rang overhead, and teenaged students began streaming from two or three of the classrooms in the corridor. John watched, intrigued in spite of himself. They were so…normal. Aside from the admittedly odd clothing—not that he could talk, thought John wryly, glancing down at his own sweeping robes—they were just the same as the kids he'd gone to school with, laughing and pushing and shoving books hastily into bags in eagerness to get to supper. Maybe it was imagination, or the result of too much time spent with Sherlock, but John thought he could read them, thought he could see the friendship and flirting and resentment flit across their features. Just a bunch of ordinary kids.

One boy swore as he trailed out of a doorway, the hem of his robes had somehow caught fire. John caught a distinctly muffled laugh and a hastily stowed wand farther down the corridor, and the smell of charred material was rising to his nostrils by the time a curly-haired brunette rolled her eyes and sent a jet of water from her wand, extinguishing the flames.

Okay, maybe slightly less than normal.

Actually, that wasn't even the strangest thing about the situation. John had relaxed his features into a disarming smile the moment the teenagers started moving down the hall toward them, but it turned out to be absolutely unnecessary: their eyes simply slid over him and Sherlock without seeming to notice them. Mystified, John turned to his friend for an explanation, but Sherlock simply inclined his head to indicate the folds of semitransparent material encompassing them. After a gaping, open-mouthed moment, John understood.

Invisibility. Okay. He could deal with invisibility.

Only when the corridor was deserted and silent again did Sherlock move. He glided soundlessly toward one of the propped-open classroom doors and John followed on his heels, doing his best to keep the cloak from sliding off his back. The classroom, though old-fashioned like the rest of the castle, appeared fairly ordinary, despite the bizarre scaled creature writhing in a tank in the corner. John managed to avert his eyes from it long enough to take note of the man behind the desk, who was rolling up scrolls of parchment while the blackboard erased itself. The man's face was kind enough, though rather too lined for his years, and he was dressed in slightly ragged brown robes. He looked up at the sound as John's sole scuffed the floor, and John, with his unerring doctor's instinct, thought his face looked pinched and slightly ill. Sherlock gave him no time to reflect on this, however.

"Evening, Remus," he announced, throwing the cloak off with a flourish. John, following suit, emerged from the waves of material in time to watch a now-familiar expression of shock slide across the professor's face. Remus, to his credit, recovered quickly.

"I am not certain…" he began politely enough, albeit with his wand pointed at Sherlock's heart. "Have we met?"

"You'd be surprised how often I've been getting that lately," remarked Sherlock flippantly. "Even from old friends."

Remus's eyes widened slightly, though his wand didn't waver. "Regulus." John had to credit him for the evenness of his tone.

"At your service."

"Not dead?"

"Evidently."

"Long story?"

"Extremely."

"Any reason you've come to call on—"

The rest of the professor's words cut off midsentence. John felt an inexplicable burst of energy heat the room and threw himself to one side, terror rising in his throat as images of sand and heat and bombed vehicles filled his mind—

When the dust cleared Remus and Sherlock were still standing, facing one another. Sherlock had his wand out and was wearing the small but genuine smile reserved for occasions in which someone else actually did something clever. John regained his feet, taking a moment to piece together what had happened.
The professor, it seemed, had fired a hex midsentence (Sherlock had taken the time to explain duels, but John still understood only vaguely how they worked) and Sherlock had blocked it somehow. Both men had barely moved a muscle. A far cry from the sort of combat John was used to.

"Really, Remus," said Sherlock conversationally, picking up the wand at his feet and tossing it back to the other man. "I'm hurt. And disappointed. I'm fifteen years out of practice, after all."

Remus gave him a grudging smile. "You can hardly blame me, Regulus, considering. What tipped you off?"

"Body language," said Sherlock. "Slight shift forward in weight. Twitch of the arm. Hardly ideal for a Defense Against the Dark Arts professor."

"Quite right," said Remus resignedly. "You'll have to forgive my shakiness, I'm not at my best right now." He glanced involuntarily toward the window, where the sun was just beginning to set.

Sherlock followed his gaze. "Of course."

"At any rate," said the professor, turning his wand absently in his hands. "You're not here to kill me. Why don't you sit down? And your friend…"

"John Watson," said John warily, declining to offer his hand. The other two seemed to have reached some sort of accord, however, so John pulled up a chair beside Sherlock.

Remus met his eye and nodded before turning back to Sherlock. "What are you doing here, Regulus?"

"Family reunion."

Remus digested this. "I presume that's not a confession."

Sherlock sighed. "I take it the headmaster hasn't yet made good on his promise."

"What promise-?" But before the words were out of Remus's mouth, something shining and silver-white streaked through the open doorway of the classroom. Sherlock pointed his wand at the door, which swung shut as the thing resolved into the shape of a large bird and spoke, shockingly, in Dumbledore's tones. John blinked hard at the space it left when it vanished, the words still reverberating through the air.

"To all staff: As of this evening, two Ministry investigators will be joining us in the search for Sirius Black. Formal introductions will be made later. In the meantime please provide Mr. Sherlock Holmes and John Watson with any assistance they require."

Remus raised an eyebrow at Sherlock. "Sherlock Holmes?"

"Regulus Black has been…indisposed."

"For fifteen years?"

"Yes."

John found himself grudgingly impressed at how well Remus was keeping up. Maybe improbable resurrections and secret double lives were standard fare in the Wizarding world.

"And why should I trust that Sirius Black's brother, given his own questionable past," Remus lingered on the last few words, "is trustworthy enough to let into the castle? Why should I believe you're on our side?"

Sherlock cocked an eyebrow. "I could ask you a very similar question."

The corner of Remus's mouth tightened. "Any friendship between your brother and myself is in the past. If Dumbledore did not believe that, I wouldn't be here."

"Ah yes, the past. You mentioned mine. And I ask you, Remus," Sherlock tilted his head. "If Dumbledore was not perfectly satisfied as to my loyalties, would I be here?"

The professor smiled tightly. "Difficult to say, where you're concerned. If ever anyone knew the castle better than the Marauders…"

"The Marauders," Sherlock sighed. "Having a secret little club is one thing, but giving yourselves such an infantile nickname…"

"I thought we weren't talking about the past?" Remus addressed the ceiling.

"How did it go, again? Best friends forever. Look how well that worked out. The midnight explorations must have been fun, though. Let's see, what was the dynamic quartet? Rat, stag, dog, and werewolf, wasn't it?"

John watched the color drain rapidly from Remus's already peaky face. "How did you…"

"My brother may have been fool enough to believe me ignorant, but I confess I expected more of you, Remus. John here knows my methods, perhaps he could explain…"

"Sherlock—sorry, Regulus—knows everything and likes nothing more than showing off to everyone within hearing radius," said John, in a passable imitation of Sherlock's usual rapid-fire deduction voice. "Get on with it, Sherlock. The poor chap's obviously exhausted. Your mere presence usually ensures that."

"John, you know my disregard for the solar system, but even I can recognize a nearly full moon."

"What's that supposed to mean?"

Sherlock nodded at Remus and John paled.

"What…you were serious…"

"Obviously."

"Of course!" John threw his hands in the air. "Can't be any scarier than you are, mate. Er…it isn't a full moon tonight, is it?" With a worried glance at the window.

"No, it's not," Remus broke in, tone strained in either irritation or amusement. "And I haven't tried to eat anyone in years."

"That's reassuring," said John, with a sort of wearied cheerfulness, wondering why he hadn't run screaming out of the castle yet. Mycroft, blast that man, was absolutely right. He was a half-crazed adrenaline junkie. Ah well. At least he had fun.

"So…let me get this straight. You and Sherlock aren't trying to kill each other."

The two men looked at each other and spoke at the same time.

"Correct," said Sherlock.

"Reserving judgment," said Remus.

John sighed. "Fair enough. I'm still doing that myself."

The disheveled professor looked at him and cracked a real smile for the first time. In that moment, each could have sworn they knew what the other was thinking. Remus voiced it.

"Madness, isn't it?"

"Pretty much, yeah," confessed John. "And, er, yours?"

"Extremely. I don't know whether James and Peter made things better or worse."

"Sorry about…"

"It's all right," said Remus. "Actually, it's not, but I've had to learn to separate the memories or I'd have gone mad myself."

John nodded sympathetically.

Sherlock glanced from one to the other, obviously irked at being the one left out, for once. "Anytime you two feel like sharing your fascinating topic of discussion." His voice dripped sarcasm.

Remus and John shared another look. "Dealing with a Black," they said in unison.

Sherlock scowled.