Disclaimer: I do not own Sherlock, Sherlock Holmes, Harry Potter, or any other character. They belong to their respective copyright owners. This is a fan-work and no profit is being made from it.

The Case Of: The Hollow Pillar

"No, just bloody- no!" John Watson snapped. "I will tolerate you having me picked up off the street and driven to some out of the way location, just to have a chat. But I will not allow you to break into my home and sit there waiting for me to come home... how the hell did you get passed the alarm anyway?"

"Please Doctor Watson," his guest responded in a way that John imagined was supposed to sound placating but couldn't hide the total lack of regret. "Your alarm system is monitored by a remote company that receives a warning, assesses the situation and then decides whether to call the authorities... and indeed which ones. Now, who do you think would have the clearance to inform them that this was not a burglary and instead a police warrant being execute on a notorious drug den?"

"Why are you here?" John growled.

"Where is my brother?" came the answer.

"How the bloody hell should I know. He told me he was off following a lead on the case we had last week. I didn't have anybody to baby-sit, so I had to stay back."

There was a silence as Mycroft Holmes seemed to stare at him, searching his face for some clue.

"It might interest you to know that my brother solved that case three days ago, culminating in a chase through a rail station and an attempt by the criminal to jump the tracks and escape; the attempt was a failure of course and the suspect is currently awaiting charges just as soon as he recovers from his injuries. Sherlock has been lying to you... again. Now, Doctor Watson, what does that tell you?"

John didn't have to think for long. He knew Sherlock. Sherlock was a habitual liar and completely indiscriminate about who he deceived in order to get the results he was after. However, if there was one thing John also knew it was that Sherlock never claimed to be on a case - except where it annoyed somebody who wanted him to investigate something else. So that left one of the famous detective's other known vices.

"Drugs," he said quietly.

He had been surprised when he had found out the Sherlock Holmes was a junkie. His surprise had not lasted long and in the years that he had grown to know the man, John had become aware that Sherlock was far from the man that John Watson portrayed to the public.

"No, not this time," Mycroft replied. He held up his hand to ward off the obvious questions. "Normally I would have only said that after having every stone overturned and every slum, drug den and underpass checked for signs of my little brother. Sherlock is not at this moment high, although it would explain a few things. I happen to know exactly where my brother is. I was hoping you would know why."

"And where is he, exactly?" John asked.

"Here!" Mycroft proclaimed, opening a laptop handed to him by his assistant. On the screen there was the footage from a security camera pointed at a bench upon which sat Sherlock Holmes.

"He's at a railway station," John deduced, the big train in the background making it obvious.

"Indeed," Mycroft agreed. "He has been there every day from early morning to late afternoon, sitting on that bench, looking neither left nor right. For a man with my brother's lack of patience, I'm sure you can see how that would be a difficult feat."

John snorted. Oh, Sherlock Holmes was a contradiction. He had an incredibly short attention span unless he was onto something or at least believed he was onto something. When he did find something though he could sit and wait for as long as he needed in order to spring the trap. He was impatient and stubborn, a sometimes-amusing combination.

"What happened to the camera?" John asked.

"I'm sorry?" the assistant... Andrea?.. asked.

"The camera flickered out and then came back on," John explained.

"It happens quite a lot on the platform," Mycroft answered. "Old wiring and faulty equipment. The odd outage is to be expected."

"Mycroft," John said softly. "You're lying again."

He was amused to see the other man attempt to act affronted before giving up and assuming his usual look.

"And how did you determine that, Doctor Watson?"

"You told me Sherlock had been in the same place for more than two days," John answered. "That means he leaves the area every night to come back here. There is no way you would have allowed a faulty camera to stop you seeing everything that was going on when you could have had a team in to replace it on the second night. Which means you either tried to replace it and failed to improve the problem or you know what the problem is." There was a pause. "You do know. This isn't about you being worried about Sherlock, you're afraid he might uncover something you already know."

"Doctor Watson - John-, it really would be in my brother's best interest not to keep hanging around in that area. I would tell him myself, but I think we both know what the result would be. And despite what he might think, I appreciate Sherlock enough to prefer him without... alterations."

"You think somebody is going to do something to stop him," John guessed.

"I know somebody is going to do something if my brother uncovers something he shouldn't or carries on making a nuisance of himself. Despite what Sherlock claims I do not run all the British government. There are some parts of it that are far outside of my authority that I could do very little to stop if they decided my brother was likely to expose them. And there are some that would take just his sitting in that spot as sufficient reason to harm him. So please, I am asking you as his friend: convince my brother to find a different obsession. Maybe a trip to one of the local drug dens would be a better alternative."

He didn't mention there were some things that could be administered to his brother that would be worse than any of the narcotics Sherlock indulged in. He didn't want to warn him that there were things they could do to Sherlock that would leave the man a drooling vegetable with no chance of recovery. The people that had drawn his brother's attention were an element over which Mycroft had very little control and as much as he wished to use his brother to find him a way to extend his reach where they were concerned, he had not yet developed a plan that would make it worth the risk.

"Sir, something's happening," Andrea said, she had been watching the events and taking notes the entire time.

John and Mycroft shifted their attention back the screen and John groaned. There was a scuffle on the platform and right in the middle of the sheer bedlam, he could make out the distinctive hat of Sherlock Holmes; surprisingly for once compared with some of those around him, Sherlock's hat did not appear too out of place.

"Is that a vulture?" he asked before realising he had asked the question.

"Oh Sherlock," Mycroft groused. "Why couldn't you just ignore it or go study street magic? Rewind the recording to where the fighting begins," he instructed.

A button was pressed, and the video returned to a previous time showing Sherlock, the bench, an otherwise empty platform and the diving barrier between Platform Nine and Platform Ten. The camera image disappeared for a short time before returning to normal except now there were more people on the platform in front of the pillar.

"Wait, how did they get there?" John asked. "And why are they walking away from that barrier?"

Indeed, if Mycroft had not known what was happening, he would have picked up on the unusual behaviour in an instant. No doubt his brother had seen it too. Perhaps that was what had drawn his brother to stake out the pillar for the last four days.

"There's no train there, but they're walking away from the track. They weren't there before the camera cut which means they either decided to walk to the centre of the pillar and then turn ninety degrees..."

"Or Doctor Watson?"

"...Or the walked out of the pillar," John finished, looking wholly unconvinced.

"You've been spending too much time around Sherlock," Mycroft announced. "You've picked up on his habit of noticing things that are inconvenient for you to know."

They continued to watch the video as the woman with the vulture for a hat led a teenage boy away and a family of redheads departed in the other direction. A girl with frizzy hair was met by what appeared to be her parents and was led off while pushing a trolley. And then a boy with glasses and messy hair was greeted by what John assumed was his family.

"Oh dear," he heard Mycroft mutter.

John frowned as he watched the interaction between the family, the large man waggled his finger in the boy's face and the boy seemed to flinch. He saw the woman turn her head and say something to her uncle who seeing only Sherlock supposedly reading whatever Sherlock was reading to cover his stake out, chose to clout the boy around the back of the head in public.

Exactly how that action led to Sherlock and the man engaging in a heated discussion was unclear, but when the camera blanked out again and returned, another group of people had appeared, and these ones seemed to be threatening both Sherlock and the boy's family. And when one of those people had brandished a stick in Sherlock's face, all hell had broken loose.


Author's note: I've read a few Sherlock and Harry Potter crossovers in the past. I've seen quite a few that involve Sherlock being hired or meeting Harry during a case. On this occasion the crossover is caused by Sherlock's inability to let something go once he gets an idea in his head. Such as a momentary glimpse of a wizard appearing or disappearing at the station that causes him to begin a stake out. And things escalate from there.
As for Mycroft... I have no problem believing that he is aware of the magical world. I also believe that he would have teams of agents capable of installing replacement cameras quickly if it meant being able to keep a close watch on Sherlock.
Perhaps the biggest problem with Harry Potter / Sherlock crossovers is timing. In this case it would be easier to say that the events are taking place after Season 4 of Sherlock and before Book 3 in Harry Potter. Consider that the events of the Harry Potter books are taking place at a much later date.