Sherlock had taught John a lot; more than he had realised. He had taught him how to re-construct several weeks, a month even, of a man's life. He had taught him how to talk to relevant parties to work out where he had been and when. It had taught him how to trace the journey of an addiction, and John was in absolutely no doubt of the significance of what he had discovered.

That Sherlock had been using drugs was clear. That he had been doing so in plain sight in order to ensure that Magnussen would be aware of what he was doing was also clear. John wished with all his heart that he could believe Sherlock's story that this was all for a case, but there were signs there that not even he could miss.

Sherlock had shut himself away from everyone other than Janine for the last four weeks. He had seen her twice a week, always on the same days. Something stopped John from prying too closely into the gritty details of Janine's relationship with Sherlock. He found that he simply didn't want to know. It felt - wrong, somehow - unsettling. He told himself that it was because he was aware that the whole thing was a fake, a ruse to get into Magnussen's office, but a niggling voice at the back of his head told him that he'd been unsettled by it from the beginning, even before he'd know the truth of it.

He had simply never considered Sherlock in that light before. Irene Adler apart, he had never seen him show the slightest sign of interest in any human being in that way - male or female. Initially, he had been convinced that Sherlock was gay and simply chose not to act on it. The care he gave to his personal appearance, his particularness when it came to his suits and shirts had led him to that conclusion. As time went on, he had come to realise that it was a uniform - more than that, it was armour. Sherlock had learnt long ago how to dress in order to achieve the result that he wanted. To be respected. To be trusted. There was almost a superstition in how he dressed, a fear that if he let his guard slip then the world would see the bubbling mass of contradictions that lay within. Like his ability to wheedle information out of people by acting a part, he dressed himself in a costume which facilitated this. His suits made him feel safe. They were his protection against the world. In the flat he would slop about in pyjama trousers and old t-shirts, often worn inside out. But outside, outside it was always the suit, and the coat, of course the coat.

Since his return, Sherlock had seemed - different. Less ethereal, more present in the real world. More mature? Perhaps, but there was something else that John had chosen to ignore. He had seemed more - sombre somehow. He still attacked cases with drive and his old single-mindedness, but the energy, the sheer joy in it, that was gone. What had happened to Sherlock in those two years away, John wondered? Sherlock had never spoken of it, had said that it was safer that way, but John couldn't help but think that the events of those two years had shaped the man that now resided in 221B Baker Street. Had it shaped his drug habit also? There was a chance, wasn't there?

John Watson knew all about psychological trauma. About the events that at the time you thought you'd dealt with, but which came back to bite you weeks, months, even years later. He knew about the flash-backs and the nightmares, about the voices that you just couldn't get out of your head. He knew the haunted look in a man's eyes that came from the knowledge of shots fired and lives taken. He had seen it in Sherlock's eyes after his return.

So if you had seen horrors beyond most people's imagining. If you had been beaten and tortured (and John had seen the marks on Sherlock's back when he had walked in on him changing one day, marks that hadn't been there two years ago). If you had been through all of that, and couldn't talk about it, what then? That would change a man, wouldn't it? Make him more serious, less joyful. And if his best friend, his only friend, was inadvertently, or perhaps deliberately, distancing himself from him, with a new love, with a new wife, then what other and damaging ways of dealing with their experiences might a man find?

These thoughts had taken John from the tube station to the alleyway leading up to the squat where he had found Sherlock and Isaac on that fateful morning. Billy Wiggins had hung up on him immediately when he had called him from Sherlock's phone, of course he had, and he hadn't replied to a call to John's own phone either. Spooked. Good. It was always better to have your potential informant on the back foot. Especially when you knew where to find them.

The squat looked more menacing in the gloom of dusk, although Billy Wiggins himself looked no less pathetic. John found himself wondering at Sherlock's choice of venue - it was a long way from Baker Street. Had that been deliberate? It fitted that he wouldn't necessarily want to bump into any of his own Homeless Network if he was going undercover, but then why not use them as a way in? Perhaps he had. And why pick a venue so close to where John himself lived? It seemed too much of a coincidence. Had Sherlock planted himself there deliberately, knowing that John was likely to pitch up to rescue Isaac sooner or later? Had he wanted John to find him? And if so why?

John walked straight past the door to the squat, and picked a spot to wait apparently playing with his phone on the far side of the skip situated outside, partly shielded from view of the door, but still with a good eye-line to it, enabling him to watch and wait.

What he did work out fairly rapidly, was that there was some kind of secret knock to gain admission. What was this, The Secret Seven for fuck's sake? Deciding there was no time like the present, he walked up to the front door, wishing he had his tyre iron with him, and knocked on the door. Billy Wiggins opened it, and when he saw John, took a step back and threw his arms up defensively. 'Don't hit me!' he said.

'I'm not going to hit you,' John's said with a sigh, walking into the hallway of the squat and shutting the door behind him. 'I just want to talk to you.'

'About what?'

'About Sherlock.'

'What about him? Haven't seen him for days. Not since that morning when you tried to break my arm,' Billy said, rubbing it, as if only just remembering that it still hurt.

'I didn't -' John started, then realising that he was on the wrong tack if he wanted to get information out of him. 'Look, I'm sorry about that, although strictly speaking I was just disarming you. How is it? Do you want me to have a look?'

'So you can break it for real this time? Not bleeding likely,' Billy said, taking another step back and pulling his sleeves down over his hands as if that would protect him. He was younger than John had first thought, although it was difficult to work out his exact age. Mid to late twenties perhaps, but the life that he had led had made him look older; the had been aged by adversity. John wondered what sort of life had brought Billy onto the streets. His work in general practice in this area meant that it wasn't too much of a deductive leap to imagine. It would involve violence, certainly, although whether to Billy or his mother was always more difficult to work out - both usually. Then there would have been the other abuse that would have driven him onto the street; neglect and emotional abuse at best, sexual abuse at worst, often from a family friend or one of mum's new partners. There would be alcohol or drugs involved somewhere along the line, nearly always. Other children may have been taken into care. Billy had probably been in and out of the care system himself. It was a story that John had heard all too often. The details varied - the names, the dates, the places, but the basic story; that was nearly always the same.

John was going about this wrong. He knew he was. He had been angry with Billy. For providing Sherlock with drugs, for giving him a place to inject poison into his veins, but he was coming at it from entirely the wrong direction.

'Billy, I need you help,' he said.