Sherlock is contacted by the daughter of entrepreneur Culverton Smith, who she claims has confessed to a murder, but she does not know who the victim was as her father has used a drug on her that inhibits memory. Sherlock deduces that her father is a serial killer and sets out to expose him, but he has returned to narcotics use since Mary's death and unable to clearly distinguish his own thoughts from reality. He confronts and attacks Smith, and John is forced to subdue him. While recovering in the hospital, Smith appears in Sherlock's room, confesses and then tries to kill him. John bursts in just in time to save Sherlock, who reveals that his behaviour up to that point was not just an elaborate ploy to expose Smith, but also fulfilling Mary's last wish for him to "Save John". Later John's therapist reveals that she is actually Sherlock's secret sister, Eurus, and has been using disguises to manipulate both Sherlock and Watson. The episode ends with Eurus firing a shot at John. Title and plot elements based loosely on the Doyle short story "The Adventure of the Dying Detective".