In the wake of their terrifying poolside face-off with Sherlock's insane arch-nemesis Moriarty, John blogs, Sherlock deduces, and the cases - and pageviews - roll in. But when a mysterious summons transports John and Sherlock to the inner chambers of Buckingham Palace, Sherlock's peevish brother Mycroft, an enigmatic government agent with "ultra" high-security clearance, explains that he needs Sherlock's help. Irene Adler, professionally known as "The Woman," has compromising photos of a young royal on her smartphone. Adler has reached out to the royals, although not to extort money or favor. A power play, Sherlock observes, enticed, and so begins his adventure with the woman whose website advises, "Know when you are beaten."

Sherlock and John visit Adler's Belgravia home, where she has prepared a greeting that would bring most men to their knees. The phone, whose contents are locked behind a passcode impenetrable even to Sherlock, soon becomes a dangerous liability to the inscrutable, incredible woman. Sherlock is left not just with the mystery of the passcode but that of his own feelings. So intense are those mysteries, and so high are their stakes, that John and even Mycroft fear this just may be the case to actually beat Sherlock.