Season 1

1. A Study in Pink

The police investigate a series of deaths related to 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 on 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, arranges a meeting with Watson and asks whether he will 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. Before Sherlock can play the cabbie's game, John shoots the cabbie from an opposite building. Before he dies, the taxicab driver reveals that " Moriarty " was his sponsor. Loosely based on the first Sherlock Holmes novel, A Study in Scarlet.

2. The Blind Banker

Sherlock is hired by an old friend to investigate a mysterious break-in at a bank in the City. He discovers that symbols spray-painted onto an office wall are a coded message intended for an employee of the bank, who is later discovered dead in his flat. The next day, a journalist is killed and the same symbols are found nearby. Sherlock and John follow a trail of clues that link the two dead men to a Chinese smuggling ring, who are trying to retrieve a valuable item that one of the dead men stole.

Sherlock eventually cracks the coded message based on Suzhou numerals and a book cipher, but not before John and his date, Sarah, are kidnapped by the criminals, who believe that John is Sherlock. Sherlock rescues them, but the leader of the gang escapes. Later, the leader of the gang is in communication with her superior, who is identified by the initial "M". She is then shot by a sniper. Loosely based on the short story " The Adventure of the Dancing Men ", the storyline also incorporates elements from other Sherlock Holmes stories; the concept of coded messages, the markings on the feet of the Black Lotus members and the plot of escaping a secret society, then being tracked to and killed in England all feature in The Valley of Fear. A murder victim being found inside a locked room, accessible only by climbing, alludes to The Sign of the Four.

3. The Great Game

Sherlock is commissioned by Mycroft to investigate the suspicious death of a government employee, who was working on a top-secret defence project: the Bruce-Partington Project. After rejecting the case and handing it over to John, Sherlock begins to be taunted by a criminal who puts his victims into explosive vests and sets Sherlock deadlines to solve the apparently unrelated cases, which include a twenty-year-old cold case involving the shoes of a drowned boy, the disappearance of a businessman, the death of a TV personality, and the assassination of a museum security guard by the " Golem ".

As Sherlock solves the cases, he finds links between them. After clearing up the original case regarding the government employee, Sherlock tries to force his unseen adversary to reveal himself. Near the end of the episode, Sherlock and "Jim Moriarty" (Andrew Scott) reach a standoff, where Jim reveals that he is responsible for the crimes. In the final seconds, Sherlock Holmes points his gun at a bomb on the floor that had been strapped to John. The episode's storyline is somewhat derived from " The Adventure of the Bruce-Partington Plans " and makes references to " The Five Orange Pips " at various points (as per the five bomb victims and electronic beeps on the phone) in addition to other works of Sir Arthur Conan Doyle.