A Study in Pink (2010)

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, Sherlock Holmes is introduced to John Watson, and the pair immediately move into a flat in Baker Street. John Watson slowly gets to know and trust Holmes despite a police officer warning him that Holmes is a psychopath and will one day be responsible for murder. Sherlock's brother Mycroft, at first not revealing his identity, kidnaps Watson and asks whether he'll spy on Holmes for money, but Watson refuses. After a series of incidents, the person responsible for the deaths, a taxicab driver, 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, who reveals as he dies that Moriarty masterminded the entire plan.