Contains spoilers for this episode and earlier episodes.
Mycroft arranges a cover story for Magnussen's death so Sherlock can resume his work while awaiting Moriarty's next move. John and Mary become parents to Rosamund, and John begins secretly exchanging messages with a woman he meets on a bus. A case involving a cabinet minister's dead son draws Sherlock's attention to a smashed bust of Margaret Thatcher. More busts from the same set are being stolen and broken.
Inside the last bust, Sherlock finds a memory stick connected to Mary's former team, AGRA. Her surviving colleague Ajay believes she betrayed their failed hostage rescue in Georgia. Mary flees, but Sherlock tracks her and John follows to Morocco, where Ajay confronts them. His account of the captors' words points towards someone in British intelligence. Sherlock first suspects Lady Smallwood, then realises that her assistant Vivian Norbury had access to the same secrets.
At the aquarium, Norbury admits exposing the rescue operation to conceal her own sale of information and silence the ambassador who discovered it. Sherlock continues provoking her despite the gun in her hand. When she fires at him, Mary steps into the shot and dies with John beside her. John blames Sherlock for breaking his promise to keep Mary safe and refuses his help. A recording Mary made before her death gives Sherlock another task: save John Watson.