Note: I own nothing but my own plot, everything else is the BBC's, Stephen Moffat/Mark Gatiss's, Arthur Conan Doyle's, or the incomparable J.K. Rowling's. I just like to play here. Not beta'd or Brit-picked.

Because, don't we need a Harry Potter/Sherlock murder mystery crossover?

Please note that I've tweaked the timeline to fit—just go with it.

#

"So, why are we going to … Little Whinging? How is there even a town named Little Whinging? Is it next to Greater Whinging? Part of Blubbering County? Bisected by the Distraught River, perhaps? Ridiculous."

John just sighed. "We're going because Lestrade asked us. He said this was right up your alley—a classic locked room mystery, clearly a murder, but with absolutely no clues or evidence left behind."

"We'll see about that," Sherlock said with a sniff. "I'm sure they missed everything of importance."

"Which is why they need you," John said.

"I suppose," Sherlock said, slumping in his seat as he stared out of the train window. "As if there will be anything left untrampled when we get there. These small town forces have even less patience and experience than Anderson."

"They knew enough to know they were out of their depth," John said, trying to be soothing. "Try not to worry so much?"

"Worry? Worry? I'm not worrying, John, just stating the likelihood that the first flat-footed officer through the door of this so-called evidence-free crime scene obliterated all the clues himself."

John just unfolded his paper. "All the more reason for them to ask for you—because you know what else is lacking in these small town forces?"

"What?"

"Sufficiently big egos," he said as he lifted the newspaper. "So it's lucky for them you were able to come."

#

Sherlock turned on his heel, taking in the spotless living room. Or, spotless aside from the dead body sprawled on the couch.

He looked around the room, taking in the family photos on the wall, the faint smell of bleach in the air—not enough to be trying to cover up a crime, just a miasma of constantly-used cleaning products.

"Who is he?" he asked Detective Evans as his attention turned to the dead man.

"Vernon Dursley. Married, one child. Neither the wife or the son are accounted for. We're looking for them now," the detective told him as he took in the florid complexion, the extra weight … the man would have been a prime candidate for a stroke, but that wouldn't explain the look of horror on the man's face.

"You said neither the wife nor son," Sherlock said, looking at the numerous photos of a large man in his 30s. "Does he live with them?"

"Yeah. His room's upstairs. According to the neighbours, he spends most of his free time out drinking with friends. Was a bit of a bully when he was younger." The other man trailed Sherlock as he went up the stairs, taking in the lack of dust along the railings, chattering as he looked into the bedrooms.

He paused at the smallest room, struck by the contradictions. It could almost be considered a second guest room, except the bed was little more than a mattress and the furnishings rough and broken. There were piles of boxes standing in the corners, but no sign that anyone ever spent any time in the room, as if they were uncomfortable … he took another look at the door jamb. "Whose room is this?"

Evans glanced at his notes. "No-one's. I mean, look at the dust…"

"Yes," snapped Sherlock. "No-one now, but someone did. See the holes in the door? There were multiple locks and, see? Bars at the window, as if someone was kept against his … her? …no, his will. Who was it? Would he have had reason to kill Dursley?"

Something moved by the bed and he bent down to look, just as a feather wafted out from underneath. "Is this … an owl feather? That doesn't make sense. What do you think, John?" he asked as his friend came to peer into the room.

John looked at the feather in his hand and then turned to look at the room with more attention than usual. He wandered over to the window, which seemed to have … were those claw marks? This was fascinating.

John was over by the desk, fingering what looked to be a quill, of all things, when the detective came back in. "According to the neighbour, the Dursleys' nephew used to live with them—a troubled boy, by all accounts, who went to St Brutus's—a school long since shut down for its cruel practices. He never came back, but we'll search to see if we can't find something on him." The man looked at the room with solemn eyes. "If they forced him to live like this, I can't say I'd be altogether surprised if he had chosen to come back for revenge … though why now?"

"Do you know the boy's name?" asked John.

"Harry Potter."

The voice came from the hallway and, Sherlock might be mistaken, but he could have sworn he saw John flinch.

"Oh yeah?" asked Detective Evans, staring at the red-headed man. "And who might you be?"

To Sherlock's surprise, the answer came from behind him as John said, "Ron? Ron Weasley?"

#

John didn't know what to think. He stood in the middle of what must have been Harry Bloody Potter's childhood room and didn't know what to think.

Like most of their class, he had known that Harry grew up in a muggle home, raised by his aunt and uncle who were horrid in ways that he never expounded on. John, like most of them, had just assumed that they couldn't really be that bad … He had never imagined bars on the windows.

He stood holding the snowy owl feather in his hand. What had the owl's name been? Hedwig? He wondered what had happened to her. He wondered what Harry was up to these days, hoping his old friend was still alive. Or, well, maybe not quite a friend, but…

He spun around at Harry's name, staring at the familiar face at the door. Older, yes, but unmistakable, even if the ginger hair had faded.

"Ron? Ron Weasley?"

The words were barely out of his mouth when he was cursing himself. He should have kept quiet, not drawn attention to himself. He wasn't part of the wizarding world anymore. Maybe he wouldn't remember. It had been twenty years, after all…

"That can't be John Watson?"

Damn it, thought John as he nodded and stepped forward. "Hey, Ron. It's been a while."

"I'd say so! Merlin, John … I can't believe it's you, mate. And you … you're doing all right?"

"Yeah, I'm fine," John said automatically, trying not to think about their audience. Their very muggle audience, that probably shouldn't know this man was Harry Potter's best friend. "You? Hermione and … everyone?"

Ron caught the warning in his eye and just nodded. "Hermione and I got married, did you know? And would you believe my sister married my best friend? They've got three kids now to our two, all in school together, causing trouble like we used to."

"I almost feel sorry for … the school. I haven't been back since, well…"

He couldn't quite meet Ron's gaze, which had cooled slightly. "Yeah, I understand…" He looked around and blinked, as if realizing where they were. "So—what are you doing here? Are you a policeman these days?"

"Me? No," said John, waving over an extremely interested Sherlock. "But I work with them often enough. This is my colleague, Sherlock Holmes. He's a consulting detective and helps out the police when they need him. When they found this apparent murder with no clues, no physical evidence, they called him. I just came along for the ride. Sherlock, this is Ron Weasley. We were friends in school."

"It's a pleasure," Sherlock said, eyes cataloguing every detail of Ron's clothes and demeanour. "John never talks about his school days. And what do you do, Mr Weasley?"

"Something of a police liaison myself, you could say, but from a special division. So—what do we have?" Ron was looking around the room almost nervously and John wondered if he had been here before.

"Mr Vernon Dursley," said Sherlock. "Dead from no obvious cause in a locked room with no signs of a break-in. There are footprints in the carpet that imply there had been other people in the room, though the prints just … stop, without approaching any of the exits. There is no sign of his wife or his son, but the door to the room next to this looks like it was forced open."

"Forced open?"

"Yes, but oddly, nothing is broken—which is odd, since the deadbolt was engaged, yet the wood frame didn't splinter when the door opened. And then there's the this room, which raises a number of questions, even though it has not been used or even entered recently."

John saw Ron's jaw tighten. "Why? What did you find?"

"This room looks not only unused, but ignored—as if the owners of the house preferred to pretend it didn't exist, even though space next door is at a premium. Not only that, the room has bars at the window that appear to have been replaced after being torn out from the outside—pulled straight out from the first floor, not down to the ground, which is curious. I'm told they had a nephew who lived here as a child, one with violent tendencies. Evans seems to feel he might be responsible."

John looked over, eyebrow raised. "But you don't, Sherlock?"

His friend shook his head, face thoughtful. "This room clearly hasn't been used in years—decades, even. The rest of the house is spotless, but you can't enter that room without kicking up dust. Why would the nephew return now to take his revenge? Why would he kill his uncle but not the aunt and cousin? Where are the aunt and cousin?"

"Maybe … something about them being family? Blood relations?" John suggested.

"Possibly," Sherlock said, cocking his head to one side. "There could be some kind of inheritance, I suppose, but I doubt it. I think the nephew … what was his name?"

"Potter."

"Right, Potter. I suspect he's involved but only peripherally. I want to find him, find out what he's doing these days because I believe that whoever committed this murder did so not because of hatred toward his uncle, but to try to influence Potter—wherever he is. The aunt and cousin might have run—or been taken."

"But," Ron said, "Why would they do that? If … if Potter hasn't been in that room, if they were mistreating him by locking him in …why would anyone think he cares enough to do something to help his aunt? Especially now, twenty years later?"

"I don't know," Sherlock said, clapping his hands. "Yet. But I'm going to find out. Come along, John!"

John watched Sherlock stride away, DI Evans behind him, and turned back to Ron. "I'll try to head him off, but, once he's on a trail, he's relentless. If … if he can't find Harry through normal channels, Ron, he's going to get suspicious."

"He's going to get suspicious, all right." Ron looked worried. "You don't have an owl, I suppose?"

"No, not since … I'm a muggle now, Ron. I don't suppose wizards have mobiles these days?"

"Not all of us, but I do—my in-laws need to reach us somehow, and as an Auror Muggle Liason, people need to call. The hard part is keeping my dad from stealing it for experiements."

That sounded all too familiar, thought John as he pulled out one of his cards. "Right, here's how you can reach me. I've got to go."

"Look, John…" Breath catching, he stopped at the door and looked back. "I'd really like to catch up …"

He could hear Sherlock calling him. "Later, Ron. Say … say hello for me." And, leaving Ron gaping, he hurried out of the room.

#

Sherlock tried to focus on the mystery—one of the more intriguing closed-door murders he'd seen. It was fascinating and altogether new … so why did he keep thinking about John?

Meeting an old school-mate of his was intriguing, yes. John barely referred to his army days, but he never mentioned his childhood, never talked about school or childhood friends. Nor did he ever hear from them, which was an odd discrepancy for such a cordial man. John was one of the friendliest people Sherlock knew. He might be very private about personal details, but he was … chummy. He enjoyed interacting with people and seemed happy to share a drink and anecdotes with just about anyone.

He knew that John chose not to talk about his time in the army. He kept in touch with some of his buddies and went for drinks from time to time, but he didn't actually talk about the war. From a psychological standpoint, Sherlock could understand that—thinking about them would be painful, talking about them even worse. He himself didn't exactly chatter about his experiences with cocaine.

No, he understood the need for privacy and the choice not to discuss painful events. But now he thought about it, it was odd, wasn't it? That John never spoke of his childhood? Never discussed friends from school?

And then there had been the reaction when he and Weasley had seen each other. Legitimate happiness at the reunion, leavened by … relief? As if neither had expected to see the other alive again, but that was patently absurd. John might have joined a dangerous profession, but that wouldn't explain why he was so relieved at Ron's survival—not to mention that of the other people mentioned.

"You can ask, Sherlock," John's voice came and Sherlock blinked, only now realizing he'd been staring.

"An old school-mate?"

John nodded. "We shared a dorm for six years, but I haven't seen him since we were 18."

"You liked him." Sherlock said, remembering the expression on John's face.

"Sure. He was fun—nowhere near the troublemaker his brothers were, not that he didn't do his share. But Fred and George … the twins' practical jokes were legendary. I bet students still talk about them … he'd have loved that."

His voice faded away, and Sherlock's brow creased. "Would have?"

"Fred died, my final year. I'd … almost forgotten."

There was something John wasn't telling him, Sherlock thought, watching the way his hand kept fisting. "An accident?"

"A fight," said John, and this time his tone was firm, as if he refused to say any more. "Anyway, we went separate ways after school, and I haven't seen Ron since."

"Odd that he would be at this crime scene," Sherlock said, testing something … he wasn't even sure what detail he'd seen that made him sure that John knew more than he was telling.

"Probably odder that we were," said John. "We don't get called out of London all that often."

"No, but…"

"But?"

"How did he get there, if he wasn't on our train this morning?"

John's laugh came just a fraction of a second late. "It's not that long a drive, Sherlock."

#

John didn't know how he was going to distract Sherlock from finding Harry, but their first stop seemed like a nice, time-consuming dead-end. "St Brutus's Secure Centre for Incurably Criminal Boys? Really?"

"That's where Potter was sent for school," Sherlock said. "They might have records of where he went afterwards."

"But … DI Evans said they had closed," John said, protesting.

"That doesn't mean the records aren't there," said Sherlock, squinting up at the ten-foot wall surrounding the complex. "We just need to break in."

John looked up, too. "Not exactly easy. Isn't there a doorbell, or something?"

Sherlock was pulling at the gates now. "Pointless if there's no-one to answer the door. Ah!" He pulled at the iron enough to make just a big enough gap for John to duck beneath the chain, and then followed, dusting off his gloves. "Child's play."

John wasn't comforted. "Wasn't that suspiciously easy for a school that needed a wall that high?"

"You said it yourself, John. They're not in business anymore." Sherlock strode across the brown remains of a lawn and said, "So, where do you suppose the admissions records are kept?"

#

"Yes, here he is," Sherlock said, hours later. "Harry James Potter. It's quite a thick file, too."

"Really?" asked John. How was that possible?

"You sound surprised. Judging by his living conditions at home, it hardly seems a stretch to believe he would be a troubled teenager." Sherlock leaned back in his chair, thinking.

"Well, no," said John, but he was mentally comparing his memory of his friend with the boy shown in these files. Harry had never talked much (at all) about his childhood, but it was true—with a background like that, you really would expect him to grow up troubled beyond normal teenage-angst. And that wasn't even calculating Voldemort into the equation.

He flipped through the file, skimming past notations about detentions and beatings, "extra discipline," whatever that was, and felt ill. Who would do this to a child?

Then he shook himself. But they hadn't. No matter what these records said, Harry had not been here. He had been at Hogwarts with John. He might not remember every minute of his Hogwarts career, but John was quite sure he remembered sharing a dorm with the Boy Who Lived. He might not have seen him in twenty years, but Harry Potter had gone to Hogwarts, not this … travesty of a school.

So then where had these records come from? He opened his mouth to ask Sherlock, but stopped himself. If he were to tell him these were faked, he would have to tell him how he knew … wouldn't he?

He studied the papers, trying to see clues beyond the false trail someone had laid (and, why?) He turned to the very back of the file and then flipped back to the front. "Do you know, it's the same handwriting for this entire file? And it looks like the same ink, too … does that seem possible to you?"

Sherlock had sat up and pulled the file from John. "No, it doesn't. That's … odd. And, look at this. See how the ink flow varies? It almost looks like it was written by a dip pen or a quill, rather than a ballpoint. Even fountain pens write more smoothly than this. The penmanship is odd, too…" His voice trailed off as he tilted the page, squinting at the surface of the dried ink, then he dropped it and pulled the next file from the box. "Ah, look! Completely different handwriting, very obviously a ballpoint pen … and a typewriter!"

"So … what does that mean?" John asked. "Someone made up a fake file on Harry Potter?"

"It looks that way. The question is why? And why such a bad fake?"

It really was, thought John. Why, indeed?

"So, if these records were faked, then Dursley lied," said Sherlock as they crawled back through the gate. "His nephew was never sent to St Brutus's, so … where did he go?"

"Some other school?" suggested John.

"Yes, but where? And why would he lie? Who created that faked file? Did he send him to school at all? Maybe they kept him locked in that room during term?"

John couldn't prevent a shudder. "That's a horrible thought. But then what happened to him when he turned seven… I mean, eighteen? Did they just kick him out?"

Sherlock was punching at his phone. "Nothing comes up on Google. Let's go back to Privet Drive and see if they have anything like adoption or guardian paperwork that might tell us more."

John nodded, even though Privet Drive was about the last place he wanted to go. Ron had said Harry was married with three kids, now, so he wasn't worried about his old friend's current state of mind, but if it were him, he wouldn't have wanted his past excavated like this—certainly not over Vernon Dursley.

He vaguely remembered a few references to his adopted family when they were in school, but Harry had always made a point to reassure John that he knew not all muggles were like that—although John's family had never exactly belonged on a postcard, either. His mother had died before he'd gotten his Hogwarts letter, and his being a wizard had just put more obstructions between him and his remaining family. Even when he'd had to stay home in seventh year, denied Hogwarts because of his "mudblood" past, relations had been awkward. The only thing that worked out that year was that, since he was forced to attend a muggle high school, he had had valid credentials to show when he joined the army.

No, he'd been away from the wizarding world for almost two decades now, and the last thing he wanted to be forced to do now was to not only investigate a magical murder, but one connected to his old dorm-mate and hero of the entire wizarding community? God, no.

#

Sherlock was intrigued. Not only did he have a locked-room mystery with an undetermined cause of death, two missing suspects-and-or-victims, but now there was this delicious twist of false school records for the victim's possibly-troubled nephew. The evidence of abuse (or at least neglect) in the boy's old room was plain, but if the boy hadn't lived there in years, why would they keep the room for him? And if they had neglected/abused him, why would they think the boy might come back?

And who had planted the false records at the school? How long ago?

No, he needed more data.

He looked at John slumped in his seat and tried to remember when they had last stopped for him to eat. It was always so inconvenient, but since Sherlock usually used the time to think about their cases, he couldn't begrudge John his sustenance … not too much, anyway.

John had seemed off ever since the crime scene. Something to do with the old school friend? Because that had been odd. And then his mood had soured even more after St Brutus's. "Why don't you want to go back to Privet Drive?" Sherlock asked. "Does it have something to do with that schoolmate of yours?"

He didn't miss the slight jump at the question. "Oh, you mean Ron? No, not really … or at least, I didn't expect to see him, but it's fine. What do you think about those records of Potter's? They were faked, but all the punishments were for hot-headed behaviour. If they were planted by Dursley's killer, wouldn't they have made Potter seem more, I don't know, calculating and cold? To come back after all this time for revenge?"

That was actually a good question, Sherlock thought, but then, John was often best at reading emotions.

"It depends on how long ago they were planted," he said. "If they date back to Potter's actual school days, they could have been a blind to prevent investigation as to where he actually was. If they're more recent and tied to the murder … you're right. Then they don't fit."

"Unless we're supposed to believe he's been a bomb waiting to go off all this time?"

"But why now? If Potter has had no interaction with his family for twenty years, what would drive him to kill his uncle now?"

"You really think that's likely?" asked John, a layer of tension in his voice.

"I don't know," said Sherlock. "It all depends on what Potter's doing now. He may have left and never looked back and be totally uninvolved. Or he may be a target. I can't know for sure until we've tracked him down."

"Right," said John. "So what's our next step?''

#

Later that night, back in London, John told Sherlock he was going out. He didn't tell him specifically that he was going to meet up with Ron, though he wouldn't put it past Sherlock to deduce that. John just hoped to avoid the questions and wanted to keep Sherlock from tagging along, because, well, that wouldn't go well at all. There were too many things they needed to talk about that a muggle couldn't hear.

Ron had invited him to the Leaky Cauldron, but while it was tempting, John didn't want to come that close to his past. He had suggested a random muggle pub, instead—not one of his regulars, in the hopes that it would take Sherlock longer to track him down if his flatmate got bored. (He really hoped Sherlock didn't get bored tonight.)

When he arrived, he wasn't surprised to find that Ron hadn't come alone.

Standing next to him as John approached the table was the saviour of the wizarding world himself.

"Harry."

#