That man being ex-Colonel Sebastian Moran, who has returned to London. There he stumbles upon an unlikely army doctor in a bar, which he does not recognise after three or so years.
Of course, the two would have a lot in common, and far more than they know, but none are willing to talk about their former flatmate to the other, which helps to not raise any suspicions about either's identity.
This can involve slash, though I do intend to have some befriending there.
And Sherlock will have to return at some point, only to find that his best friend has 'replaced' him with an assassin.