Mycroft has had the path of his life planned out from a very early age. It's important to look normal for the inevitable future background checks, so he's scheduled himself a brief period of teenage rebellion: time to visit a disreputable pub outside his usual haunts and get involved with an unsuitable boy. It might be a little distasteful, but it's all a necessary step in the plan.

Except, to his vague alarm, it turns out he actually quite likes the beer and the music... and he really likes the unsuitable boy.