A woman looked outside of a window, alerted by the clanging sound of bins getting knocked over on the street, followed by shouts and footsteps echoing through the empty alleys. "Don't call the police, get inside!", her husband said. "It's three in the morning, it's probably some wanker getting pissed".
The noises kept getting fainter and fainter, until they were just a memory in the woman's head, but someone – a man – kept running nevertheless: a pang in his chest as his heart beat wildly and pulsed in his ears; the drizzle didn't help, the pavement was slippery and water blended with the sweat trickling down his forehead.

He wasn't running for the sake of it (who does that at three in the morning?), he wasn't late to a meeting, he wasn't scared or anything like that and he wasn't being chased, for that matter. On the contrary.
You could call him a vigilante, if you want, but he'd probably glare at you with one of his reproaching looks: he didn't like that word, in his mind it was always linked to some loser who runs around town with a cape fluttering around his neck and hand-stitched by his grandmother, following random people and getting restraining orders.
No, he wasn't like that, he didn't even do that on a daily basis, it just happened some weeks ago for the first time: you see a man climbing out of a window with a packed shoulder bag in the middle of the night and what do you think? Reverse Santa? Yes, probably, but that's what normal people call "a robbery", especially if you see said man running right after touching the ground. That's a dead giveaway.
And if you're a sane person, if you put your well-being before some stolen jewelry, then you call the police: you give them your name, you tell them what you saw and you go to sleep content and happy to have helped someone.
But we're not talking about a normal person here, we're talking about someone who saw this and ran after the criminal: he chased him for a few blocks, before tackling him down and tying him up at a lamppost with his belt.
And he loved it. Actually, more than that, he felt alive, dynamic, he felt his blood running through his veins and his lungs expand with air; every inch of his skin was responsive, every nerve in his body was sending inputs to his brain that could process only one thought: again.

This time, the one with the bins and the rain and the curses muttered under his breath, this time was his eleventh. It happened before, usually for low rated crime (he's not a superhero, for God's sake), sometimes he just stopped some punks from beating up a kid for a couple of pounds, other times he followed people with suspicious behaviours but nothing like a good old-fashioned chase.
He turned a corner and saw the outline of the man he was following, running under the dim lights of an alley somewhere near Edgware Road: he felt his phone vibrating in his pocket and the weight of his gun in the back one.
It was a cold winter night and a strangely silent one, so after a while his ears were able to discern their rhythm, to make out the sound of his soles on the ground and the one of his opponent. It comforted him in a strange way, made him feel like he had the upper hand, so when he heard the sound of a third stride getting closer, he panicked, stopped and turned around, losing sight of the target for a moment: behind him only darkness and flickering lights.
It was just a matter of seconds, and when he started running again he finally saw someone getting closer to him.

Shit.

"Shit" was all his brain could muster. Did he stop? No, he did not, because in that moment he thought it couldn't get any better than that: his heart was thumping in his temples, his legs regained strength and a smirk appeared on his face. That was what he wanted, what he needed to get through his boring life, to go to bed at night, lie down beside his fiancée and think "I'm okay with this".
He reached behind him and placed a hand on his gun as the stranger got closer to him, running just as fast and just as determined to reach him.

Was this the guy I was following? Did he just turn while I was distracted and decided that attack is the better defence?

He mumbled something and prepared himself for the last couple of steps but as the man stopped before him he froze: he held himself against the wall as the gun fell on the ground. Their ragged breathing filled up the air and the man in front of him slowly pulled down his hoodie.

"J…John?"
"…Sherlock?"

A/N: I don't know, I had this idea on how the next season should start and here I am. Don't really know where this is going. We'll see! By the way, this is not beta'd, so you'll have to be patient with me.