There are two dozen tenants listed at the entrance of the building opposite St Bart's Hospital. A gloved finger is moving slowly down the names, past "Winkelman's Theatrical Tailors", past "Russian Enterprise Club", past "The Fontanella Ballet School", until it finds the one it seeks and presses the button.

"Bankside Rehearsal Rooms." A woman's voice, sing-song helpful, coming through the tiny speaker.

"Brymer. Clarinet practice. I'm down for a full hour."

"Oh, yes. Fourth floor, turn left when you get out the lift. Room 7 is free. Please sign out when you leave, and there's no smoking."

"Gotcha"

Rubber-soled shoes heading for the lift, squeaking slightly on polished tiles. Ancient cage-lift rattling skywards. 1..2..3..4. Clunk. Gloved hands slide back the gate. Shoes exit the lift, pause for half a minute, but don't take the corridor. Shoes take the stairs, heading skywards.

The sniper at the third-floor window of Bart's Hospital tracks John Watson as he steps uncertainly along the pavement, his phone pressed to his ear. He steps out onto the road, looking around him, bewildered, talking all the while. Finally, he swivels round and looks up. The sniper draws a bead on him, setting up for a clean head-shot, and waits. With one eye shut tight and the other clamped to the scope, he is in no position to catch a slight movement on the roof of the building opposite, nor to see or feel the red dot creeping up his clothing, up his neck, over the stock of his rifle, until it stops right where his nose meets his forehead, locked into place like a third eye.

"Sherlock!" John's despairing cry ricochets off the tall buildings, loud enough to cause the pigeons to rise and flutter. The sniper doesn't move, and neither does the red dot.

There is a flurry of activity down on the street; John Watson breaks into a run, only to collide with a cyclist coming in hard from his right. He hits the ground. The sniper's phone rings once. He draws back from the window and begins rapidly unloading and dismantling his rifle. The red dot vanishes.

Ten minutes pass, while the street fills up with flashing lights and yellow-jacketed police officers. Unnoticed, or unremarked, two people leave the scene. One steps out of the delivery entrance of Bart's with a tool-bag slung over his shoulder, heading for an electrician's van. The other comes through the front entrance of the building opposite carrying a clarinet case, ducks under the police tape, and thumbs the keypad of a mobile phone.

"No football today. Match cancelled."

"I know. Rather good, though, wasn't it? I'm told he came down like a great black bat."

"So. Are we done?"

"In what sense?"

"You know what I mean, Mycroft. Are we quits?"

"Well, perhaps not quite yet."

"Don't play games. We had a deal. If the caper goes tits-up, take out the shooter. Otherwise do nothing."

"But that wasn't the deal. At best it's Part I of the deal. We're not 'done', as you put it, unless and until. Do I have to spell it out?"

A pause. Mycroft continues.

"There are many ways to save John Watson. What I have in mind for you now is something a little different. I want you to get alongside him, if you know what I mean."

"No, I don't know what you mean."

"Come now. His best friend has just topped himself after branding himself a fraud. Poor John's all alone again. Give him a couple of months and he'll
be ripe for plucking."

"That's not my area. You need someone else for that. There are specialists."

"You're too modest, Mary. He'll gravitate to you, because he'll see you for what you are: a dangerous little elf. But he won't know it, because he doesn't know his own cravings. That's the beauty of it, you see. John will get what he needs without knowing he needs it."

"Mycroft, how did you make it to adulthood without getting a mullet shoved down your throat?"

"Goodbye, Mary. I'll be in touch. You may rely on it."

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