Sherlock Holmes has a sister. One he hasn't told anyone but John about. When he revieves a call from his mother in the dead of night, explaining that something bad has happened to her, Sherlock and John go to investigate. Jemma Simmons is recovering from an alien virus and an unplanned skydive. How will seeing her favourite brother help her cope with the nightmares of the jump?

While a mysterious murderer is rampaging around London, Sherlock and Watson encounter a strange but cheerful teenager who might be the killer. At the same time Mycroft meets Agent Coulson who is trying to keep London from being caught in the crossfire of two feuding, well armed twins.