Wounded Afghan veteran Dr. John Watson (Martin Freeman) meets brilliant consulting detective Sherlock Holmes (Benedict Cumberbatch) and moves into a flat on 221B Baker Street with him. Their landlady, Mrs. Hudson (Una Stubbs), acts as a de facto housekeeper for them. Sherlock's first case, "A Study in Pink", sees him apprehend cabbi-turned-serial killer Jeff Hope (Phil Davis). Their confrontation escalates to the point that John, who's followed Sherlock unnoticed to the scene, shoots the cabbie out of fear for Sherlock's life. However, Sherlock manages to extract from the dying cabbie that he was being sponsored in his crimes by a figure called Moriarty, who is apparently a "fan" of Sherlock's.
The case seals Sherlock and John's friendship. In their next case, "The Blind Banker", they uncover a Chinese smuggling ring, which is again implied to have been linked to Moriarty.
"The Great Game" sees Sherlock being baited into solving a series of "puzzles" by Moriarty, who is as yet unseen. Each puzzle involves solving an undetected crime, with the additional incentive being that failure to solve the crime within a set time will lead to the death of an innocent bystander. Simultaneously, Sherlock works on recovering the missing Bruce-Partington Plans for his older brother Mycroft (Mark Gatiss), who is a powerful figure in the British government. The series ends with Sherlock encountering Moriarty (Andrew Scott) by a swimming pool, where Moriarty warns Sherlock that he will destroy him.
Moriarty's pool-side encounter with Sherlock ends abruptly when Moriarty receives a cellphone call from dominatrix Irene Adler (Lara Pulver). In "A Scandal in Belgravia", Sherlock is assigned by Buckingham Palace to recover some damaging photographs in Irene's camera phone. Sherlock meets Irene but is outwitted and fails to retrieve her phone. She becomes "the Woman" in his mind.
Months later, Irene meets Sherlock once more and tricks him into decoding a message for her, which turns out to be a matter of national security. Irene passes on the message to Moriarty and then blackmails Mycroft into granting her a list of demands or face Sherlock's exposure as a security leak. However, at the last moment, Sherlock figures out the password to her camera phone and crushes her bargaining power. The defeated Irene fakes her death once more with Sherlock's help to escape the wrath of the British government and enter a life of anonymity.
Sherlock and John travel to Dartmoor in "The Hounds of Baskerville" to solve the mystery of a gigantic hound that apparently caused the death of client Henry Knight's father years ago. The "Hound" turns out to be a hallucinogenic drug that was being secretly developed as a chemical weapon at the nearby military base of Baskerville. The man responsible, Dr. Frank Mortimer, is killed in a land mine explosion.