A particularly sadistic killer had been targeting street performers in the Underground for several days now, and Sherlock's investigation had hit a wall. He was driving John crazy, pacing, ranting, and alternating between flurries of activity and fits of listless boredom.
"I need to find a way for them to trust me, John. Get them to open up."
John pursed his lips, surely the bloody obvious solution hadn't escaped Sherlock. He stared pointedly at the violin resting on the arm of the square armchair.
"You could just go undercover?"
A cloud passed over Sherlock's face, he was clearly irritated with himself for having overlooked it. He walked over to the violin and picked it up, cradling it delicately under his chin and plucking out a few notes.
"Thank you, John. Sometimes the more mundane, obvious solutions slip right by me."
"You've got quite a knack for turning a compliment into an insult, you know that? We'll have to work on your clothing though, your usual posh toff uniform is going to look a bit out of place down there."
About an hour later, having raided their wardrobes, Sherlock stood in the kitchen in a pair of ratty jeans, some trainers dug up from the back of his own closet, and one of John's frayed button-downs, looking every part the starving busker.