The police investigate the deaths of a series of people who all appear to have committed suicide by taking a poisonous pill. They turn to their unofficial consultant, Sherlock Holmes, who deduces various elements pointing to a serial killer. Meanwhile, Holmes is introduced to John Watson, a former soldier who served in Afghanistan, and the pair immediately move into a flat in Baker Street. John Watson slowly gets to know and trust Sherlock despite police officer Sally Donovan (Vinette Robinson) warning him that Holmes is a psychopath and will one day be responsible for murder. Sherlock's brother Mycroft (Mark Gatiss), at first not revealing his identity, kidnaps Watson and asks whether he'll spy on Sherlock for money, but John refuses. After a series of incidents, the person responsible for the deaths, a taxicab driver (Phil Davis), reveals that his victims took their own lives by playing a game of Russian roulette with two pills: one fatally poisonous, the other safe. Watson shoots the cabbie and runs away. As he dies, the taxicab driver reveals that Moriarty was his sponsor.