Here are summaries (warning—there are major spoilers) of episodes in season four.
Season Four (2017)
Episode 1: The Six Thatchers
Sherlock returns to England after Moriarty’s apparent reappearance at the end of Season Three. Rather than prosecute Sherlock for killing Charles Augustus Magnussen, the government alters the official account of Magnussen’s death. Sherlock is convinced that Moriarty has arranged something to happen after his death and waits for his next move.
Meanwhile, Watson and Mary become parents to a daughter, Rosamund, and Sherlock becomes her godfather.
Sherlock takes on a series of cases while watching for signs of Moriarty. One involves the mysterious death of Charlie Welsborough, the son of a government minister, whose body is found inside a car outside his parents’ home. While investigating, Sherlock becomes intrigued by an apparently unrelated detail: someone has smashed a bust of Margaret Thatcher.
More Thatcher busts are destroyed, and Sherlock follows the person responsible. He discovers that one of the busts contains a memory stick marked A.G.R.A.—the same name connected to Mary’s former life as a covert operative. The man destroying the busts is Ajay, one of Mary’s former teammates. Years earlier, the A.G.R.A. team had been sent on a rescue mission at the British embassy in Tbilisi. The operation went disastrously wrong, and Ajay was captured and tortured. During his captivity, he repeatedly heard that “the English woman” had betrayed the team, leading him to believe Mary was responsible.
Mary leaves England to draw danger away from Watson and Sherlock, but Sherlock tracks her down. Ajay eventually confronts them, still believing Mary betrayed A.G.R.A., but he is killed before the truth can be fully uncovered.
Sherlock realizes that the clue Ajay heard during his captivity was not a reference to Mary. It leads instead to Vivian Norbury, Lady Smallwood’s secretary. Norbury had been selling government secrets, and when the British ambassador in Tbilisi discovered what she was doing, she arranged the attack that destroyed the A.G.R.A. mission.
Sherlock confronts Norbury at the London Aquarium. As he exposes what she has done, he taunts her about being an ordinary person who managed to fool everyone. Norbury pulls out a gun and fires at Sherlock.
Mary jumps in front of the bullet. She dies in Watson’s arms, telling him that she loves him and thanking Sherlock for the life she had with them. Watson is devastated and blames Sherlock, who had promised to protect Mary.
Sherlock later receives a video message that Mary recorded before her death. In it, she gives him one final case: save John Watson.
Episode 2: The Lying Detective
After Mary’s death, Watson remains estranged from Sherlock and struggles with his grief. Sherlock, meanwhile, appears to be spiraling out of control, abusing drugs and behaving increasingly erratically.
A woman who identifies herself as Faith Smith visits Sherlock. She is the daughter of Culverton Smith, a wealthy businessman, philanthropist, and public figure. She tells Sherlock that her father once gathered several people together and confessed that he was going to murder someone. Before doing so, however, he drugged everyone in the room so that they would forget what he had said.
Faith wrote down fragments of her father’s confession before the drug took effect, but she cannot remember who Smith intended to kill.
Sherlock becomes obsessed with the case and concludes that Culverton Smith is not planning a single murder—he is a serial killer.
Sherlock publicly accuses Smith, but his drug use and increasingly unstable behavior make his claims difficult to believe. Mrs. Hudson eventually takes matters into her own hands, forcing Sherlock into her car and delivering him to Watson.
Watson discovers that Sherlock’s apparent breakdown is at least partly a plan. Before her death, Mary instructed Sherlock to save Watson, and Sherlock believes that putting himself in danger will force Watson to come to his rescue.
Sherlock ends up as a patient in Culverton Smith’s hospital. Alone with Sherlock, Smith admits that he is a serial killer and begins suffocating him.
Watson arrives in time to save Sherlock. Smith’s confession has been recorded, providing the evidence needed to arrest him.
Sherlock and Watson begin to repair their friendship. Watson admits that before Mary died, he had been exchanging messages with another woman and feels guilty for having emotionally betrayed his wife. Sherlock helps him confront his grief, and Watson finally begins to accept Mary’s death.
But the case contains another mystery. The real Faith Smith later appears, and Sherlock realizes that she is not the woman who originally came to Baker Street.
Watson then visits his therapist—the same therapist he has been seeing since Mary’s death. She reveals that she was also the woman Watson met on a bus and secretly exchanged messages with. And she was the woman who posed as Faith Smith. She is actually Eurus Holmes: Sherlock and Mycroft have a sister.
Eurus raises a gun and shoots Watson.
Episode 3: The Final Problem
Watson survives—Eurus shot him with a tranquilizer rather than a bullet—and he and Sherlock force Mycroft to reveal the truth about the Holmes family’s long-buried secret.
Eurus is Sherlock and Mycroft’s younger sister. Even as a child, she possessed an extraordinary intellect, but she was also dangerously detached from other people. Sherlock has almost completely erased her from his memory.
Mycroft explains that when Sherlock was young, Eurus made his beloved “Redbeard” disappear and refused to reveal what had happened to him. She later burned down the family home and was taken away. Sherlock’s parents were eventually told that Eurus had died in another fire, but that was a lie.
Eurus was actually transferred to Sherrinford, a secret, heavily secured institution on an island, where Mycroft has kept her existence hidden for years.
Sherlock, Watson, and Mycroft travel to Sherrinford, only to discover that Eurus has manipulated the people responsible for guarding her and effectively taken control of the institution.
She traps the three men and forces them through a series of cruel psychological tests. They face impossible choices involving innocent lives and one another as Eurus studies Sherlock’s emotional responses.
Moriarty also appears on video screens throughout the ordeal. He is still dead: Mycroft let Eurus meet him years earlier as a reward for helping the government, and Moriarty recorded messages for her to use as part of her future game.
Eurus eventually forces Sherlock to choose between killing Watson and killing Mycroft. Sherlock refuses to play by her rules and instead threatens to kill himself. Eurus stops him, and the game shifts again.
Sherlock awakens at Musgrave, the Holmes family’s childhood home, while Watson is trapped at the bottom of a well that is slowly filling with water. The well finally unlocks Sherlock’s suppressed childhood memories. Redbeard was never a dog. He was Victor Trevor, Sherlock’s childhood best friend. Eurus was jealous of their friendship and trapped Victor in the well, where he died. Sherlock was so traumatized that he transformed the memory of Victor into that of a beloved dog named Redbeard and eventually erased Eurus herself from his memory.
Throughout Eurus’s game, Sherlock also speaks by telephone to a frightened young girl who appears to be trapped alone on an airplane while everyone around her sleeps. He finally realizes that there is no plane and no stranded child. The girl is Eurus.
The story is a metaphor for her own isolation: she is lost, frightened, and unable to understand how to connect with the people around her.
Sherlock finds Eurus at the family home. Rather than trying to defeat her, he offers her the emotional connection she has been unable to ask for. Her game ends, and Watson is rescued from the well. Eurus is returned to Sherrinford. Sherlock continues to visit her, and although she rarely speaks, the siblings begin communicating by playing music together.
Back at Baker Street, Sherlock and Watson rebuild 221B. In a final video message, Mary reflects on who they are and what they mean to each other. Sherlock Holmes and Dr. John Watson return to solving crimes together at Baker Street—the detective and the doctor, back where they belong.