The first time John sees the graffiti, it's a week after the funeral and he's not quite sure he hasn't gone insane. After all, he only glimpses it as the taxi rushes past, and he's sure there are plenty of other words that could look like 'I believe in Sherlock Holmes' at high speed.

Two days later, he catches sight of another one. 'Moriarty was real', it says, and he stops, stares, runs his fingers over chipped concrete and wonders if madness usually expresses itself through thin smears of paint under a bridge somewhere in Hammersmith.

Three days after that, he has to admit that it isn't just him. There's graffiti on the Underground, scratched into the side of the Houses of Parliament (and what will Mycroft make of that?), in the back rooms of pubs and in bathroom stalls. The capital is occupied by a silent army with only one message: we believe in Sherlock Holmes.

John's heart aches to see it, because he is barely adjusting to a world without Sherlock, a world in which the brilliant, beautiful man he knew was a delusional liar. It took everything he had to convince himself that not even Sherlock could come back from the dead. He doesn't know what it would do to him if he turned out to be wrong. He doesn't dare to hope.

It takes six weeks of soul-searching before John allows himself to act. As statements go, it's negligible, almost cowardly: a few Sharpie-d words in a hospital toilet. But it's enough. It connects him with thousands of others across London – across the world, in fact; reports are beginning to come in about strange graffiti in Melbourne, in Vienna, in Singapore. And it affirms a truth he never dared to utter.

He believes in Sherlock Holmes.