Mycroft hires Sherlock and John to retrieve compromising photos of a minor royal, which are held on Irene Adler's (Lara Pulver) camera phone, a ruthless and brilliant dominatrix who also trades in classified information extracted from her rich and powerful clients. Sherlock obtains Adler's phone, but discovers it is booby-trapped and requires a code to disarm it. When Adler discovers that the CIA are on her trail, she disappears and is then apparently killed, only to reappear to ask John to get her camera phone back from Sherlock. Weeks later when the coast is clear, Adler tricks Sherlock into deciphering a coded message on her phone which she obtained from another well-connected client. She sends the message to Moriarty, who in turn uses it to foil a British counter-terror operation. She almost succeeds in blackmailing Mycroft, but Sherlock finally cracks the password for the phone, leaving Adler without the protection she needs to survive. The episode concludes as Mycroft tells John that she has been killed by a terrorist group in Pakistan. This is also untrue: Sherlock helped her escape when she was on the brink of death.
Based on the short story "A Scandal in Bohemia".