Mycroft stood on the steps leading up to the sitting room of 221B Baker Street, both hands resting on the handle of his umbrella like the pommel of a broadsword. "Are you aware of Sherlock's history of addiction?" he drawled.
Doctor Watson stood at the top of the stairs, pleasantly and implacably blocking the way. "Yeah, Lestrade mentioned it. Why? I can't imagine you have a warrant."
"I hardly think it would be in your best interests to make this an official visit, Doctor. If your flat is found to contain Class A restricted substances, it might endanger your medical license."
Watson's smile flickered to something far less friendly. Mycroft instantly recognized his error; given Ms Watson's alcoholism and Doctor Watson's stubbornness, he ought to have used honey rather than vinegar. Time pressure was making him clumsy.
"Yeah, now I'm definitely not letting you in my home. You can try again when Sherlock gets back."
Watson began to turn away. Mycroft reached out and grabbed his wrist. Watson went very still.
Mycroft released him and took one step back, down the stairs. He controlled an impatient impulse to have Watson removed from the flat. Should Sherlock return to find the good doctor missing, it would likely send him completely off the rails. "By then it would be too late. Please, give me a moment to explain."
Mycroft marshaled his arguments. "Sherlock has been doing well, since you moved in. And when he is well, he likes to purchase small amounts of drugs and secret them about his home, as proof that he does not need them. Unfortunately, when he goes too long between cases, or when a case goes poorly, such as the McCullough kidnapping, he falls into a rather black mood. There are certain danger signs, and I have observed them in Sherlock today. I am having my brother tailed, ostentatiously – no one will sell to him. But if he arrives before I have cleared out all of his little drugs caches, he will use tonight."
Watson looked Mycroft in the eye, his features eloquently telegraphing scepticism.
Apparently some expression of sentiment was required at this point. If that was the best way to move Watson, he would wield it as a weapon. Mycroft allowed his normal reserve to crack as he summoned the memory of Sherlock's over-dose, of every time he had found his little brother endangering his own life and sanity, of the god-damned lists that were Sherlock's sole, grudging concession in Mycroft's ceaseless battle to keep him alive.
Watson's face softened. "Come in," he said quietly. "Show me where to look. Then, if Sherlock has another one of these 'danger nights', I can take care of it for you."
Mycroft felt a spike of gratitude rising in his throat, hot behind his eyes. It took only a moment to re-establish his self-control. He nodded sharply and pushed his way past Watson into the flat.