Before Sherlock can play the cabbie's game, John shoots the cabbie from an opposite building. 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.

Sherlock's brother Mycroft (Mark Gatiss), at first not revealing his identity, arranges a meeting with Watson and asks whether he will spy on Sherlock for money, but John refuses. 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. 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. They turn to their unofficial consultant, Sherlock Holmes, who deduces various elements pointing to a serial killer. The police investigate a series of deaths related to people who all appear to have committed suicide by taking a poisonous pill. As to the future of the series, Gatiss stated that due to the conflicting schedules of Cumberbatch and Freeman, a potential fifth season is still up in the air.

As of 15 January 2017, 13 episodes of Sherlock have aired, including one special, concluding the fourth series. In January 2014, Moffat confirmed that the series would have a fourth series, and that a fifth series had been plotted.

Other recurring roles include Una Stubbs as Mrs Hudson, Holmes and Watson's landlady and co-creator Mark Gatiss as Sherlock's brother, Mycroft Holmes. Molly Hooper (Louise Brealey), a pathologist at Bart's Hospital occasionally assists Holmes in his cases. Although the series depicts a variety of crimes and perpetrators, Holmes' conflict with his archenemyJim Moriarty (Andrew Scott) is a recurring feature. Holmes is assisted by his flatmate, Dr John Watson, who has returned from military service in Afghanistan. Sherlock depicts 'consulting detective' Holmes, assisting the Metropolitan Police Service, primarily Detective Inspector Greg Lestrade (Rupert Graves), in solving various crimes.

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