Contains spoilers for this episode and earlier episodes.
Sherlock forces Mycroft to explain the sister hidden from him since childhood. Eurus was confined at Sherrinford after dangerous acts against her family, and Sherlock has rewritten his memories of her. A grenade delivered to Baker Street drives the three men from the flat. Sherlock and John travel with Mycroft to the island prison, where they discover that Eurus has gained control of the staff and undermined the safeguards around her confinement.
Eurus subjects the men to tests that demand killings and emotional betrayals, using recorded messages Moriarty made before his death. Sherlock also hears a frightened girl calling from an aeroplane whose passengers will not wake. One ordeal forces him to make Molly confess her love under a false threat to her life. When Eurus orders him to choose between shooting John and Mycroft, Sherlock threatens to kill himself and the tests end in tranquilliser darts.
Sherlock wakes at the ruined family home while John is trapped in a filling well. The remains there reveal that Redbeard was Sherlock's childhood friend Victor Trevor, whom Eurus killed, rather than the dog he remembers. Sherlock deciphers her old song and finds her, recognising that the girl on the plane expresses her own isolation. He reaches her and saves John. Eurus returns to Sherrinford, where Sherlock visits to play violin with her. John and Sherlock restore Baker Street and resume their cases.