The Sexual Awakening of Sherlock Holmes and Dr. John Watson (слэш)

Sherlock owes Mycroft a favor. Mycroft calls in that favor by offering Sherlock's consulting services in a charity auction. Sherlock and John soon find themselves at the country manor of Mrs. Ives-Patton Smarmington III - not very coincidentally a long-time friend of Sherlock's mother - where they are reluctant participants in her Murder Mystery Weekend. It's a play within a play for Sherlock and John, and their roles for the weekend event bleed over into their real lives, waking the sleeping dragons within.
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In which John learns that Sherlock owing Mycroft a favor is very suspect, and Sherlock has a very bad idea.