Following a bizarre stand-off with master criminal Moriarty, ended when the villain responds to a phone call, Sherlock interrupts his investigation of a rural slaying when summoned to Buckingham Palace.

Dominatrix Irene Adler has incriminating photos of a royal princess which Sherlock is engaged to retrieve. However, having engineered a meeting with Irene, Sherlock realizes that she has far more dangerous evidence in her possession, sought by rogue CIA agents, and which cause her to fake her death and pass the facts, encrypted in her camera phone, to Sherlock.

Having deciphered the explosive result and discovered a government plot, Sherlock has to consider if Irene can be trusted and indeed if she is in league with Moriarty.