Season synopsis
Contains spoilers for this season and earlier seasons.
Mary and John become parents to Rosamund, but a string of smashed Margaret Thatcher busts draws Mary’s former intelligence team back into their lives. A surviving colleague believes she betrayed a failed rescue mission. Sherlock discovers that Vivian Norbury actually leaked the operation to protect her sale of state secrets. At the aquarium confrontation, Norbury fires at Sherlock and Mary intercepts the bullet, dying in front of John. John blames Sherlock for failing to protect her and cuts him off, while a message Mary recorded before her death asks Sherlock to save her husband.
Sherlock’s pursuit of hospital benefactor Culverton Smith draws John back into his orbit. Smith uses his privileged access to murder patients, and Sherlock puts himself in danger to obtain a confession. John reaches him before Smith can suffocate him; a recorder hidden in John’s walking stick captures the admission. The friends begin to reconcile, with John also admitting his secret texting relationship during his marriage. His therapist then reveals that she was both that correspondent and the woman who had approached Sherlock posing as Smith’s daughter. She is Eurus, the Holmes brothers’ hidden sister.
At the island prison Sherrinford, Eurus has manipulated the staff and takes control, forcing Sherlock, John and Mycroft through deadly tests. Moriarty’s taunting messages were recorded before his death for her use. The ordeal uncovers Sherlock’s buried childhood trauma: Redbeard was his friend Victor Trevor, whom Eurus killed, rather than the dog he remembers. As John faces drowning in a well, Sherlock recognises the frightened girl supposedly calling from an aircraft as Eurus’s expression of isolation. He finds her and saves John. Eurus returns to custody, where Sherlock visits to play violin with her. John and Sherlock rebuild Baker Street and return to solving cases.