He'd been kicked out of his flat—again. According to his landlord, it was the unknown smell that was the straw that broke the camel's back. Sherlock knew better. It was everything. Now he tinkered with chemicals in one of the laboratories at Bart's, thinking hard about what to do, and nothing was coming. He had nowhere to stay tonight.
"No one wants me for a flatmate," he'd told Mike Stamford earlier after the bespeckled man pointed out he could get a flatshare. "Hardly surprising."
Now the fumes were starting to get to Sherlock's head, and he smirked slightly. It wasn't his old high that kept him from clawing his brain out, but it was a similar dizziness. The door opened, and two men were standing there, one of whom was Stamford. Sherlock stared at the other man, noticing he was a war veteran, and asked if he'd served in Afghanistan or Iraq. Stamford smiled before leaving, the other man—John Watson—remaining. Sherlock asked him to come around the next morning to check out the flat he'd had his eye on. There was something about this man, something…right.
He'd had to stuff things in his potential next flat. It wasn't that he couldn't afford it now, he just sort of didn't want to live alone. He recognized the need for a grounding force, someone to tell him when he was liable to wind up on the streets again. And there John was. And he proved himself in more ways than one that week, saving his life, telling Sherlock when he was being A Bit Not Good.
Mycroft came around once or twice, but always left with a frown, concerned for his brother's mental health. In Mycroft's mind, Sherlock was unstable, and had been for decades, ever since his near-death experience from an overdose of cocaine. He'd always been troubled, but that was when it had escalated. The younger Holmes didn't make friends. He couldn't keep a job, didn't want to. Rarely ate. Barely slept. Spent days at a time in complete silence followed by days of never shutting his mouth. Had no sense of self-preservation. But John seemed to help. John told him to eat and sleep. John helped him find jobs. John who was always out, working or getting the shopping when Mycroft visited. Sherlock didn't understand the look Mrs. Hudson gave Mycroft as he left.
Time went on, and Sherlock fleshed out a bit, physically. John's insistence that he eat and sleep at regular intervals kept him from starving, and Sherlock was generally becoming a more well-centered person. But then the Fall came, and he had to leave Baker Street, had to leave John, had to leave his only friend out of necessity.
He returned after those nightmare three years to find the flat empty. He hadn't expected John to stay, not when Sherlock was "dead"—sentiment, Sherlock understood, was a powerful thing, and it could repel just as well as attract. So he sat and waited for a while, no answer on John's phone, but that was alright, three years was a long time, and people change numbers at least that often. But after two weeks of waiting for John, he decided to ask Mrs. Hudson for his whereabouts.
"Sherlock…there never was a John Watson."
"He never existed."
"Of course he did. He moved in with me. He blogged about me. How else do you think I got cases?"
Mrs. Hudson went inside and called Mycroft. Sherlock couldn't get a straight answer from her. When Mycroft arrived, he went straight upstairs.
"Sherlock, I understand this is hard for you to hear."
"Did Moriarty's men put you up to this?"
"Listen to me, Sherlock. There never was a John. You've always lived alone. Your clients came to you from your website. I've done my best to hope that you would…outgrow this imaginary friend your mind thinks it needed, but—"
"He. Was. Real."
"No, Sherlock, he wasn't."
Something in Mycroft's eyes told Sherlock the whole story. The grief and anguish of seeing a sibling unable to distinguish between reality and his own imaginings. The desperation that it was a passing thing and that during the three years, he would have understood what was going on in his head. The denial of the fact that his brother was mad starting to dissolve.
There never was a John Watson.