Contains spoilers for this episode and earlier episodes.
John struggles with Mary's death and sees her in conversations that take place inside his own mind. Sherlock, increasingly ill through drug use, receives a visitor claiming to be Faith, the daughter of wealthy hospital benefactor Culverton Smith. She remembers fragments of her father's admission that he needs to kill someone. After walking with her through London, Sherlock decides the missing word was anyone: Smith kills because he enjoys it.
Mrs Hudson brings Sherlock to John, and the investigation takes them into Smith's hospital. Smith turns the public accusation into a publicity joke while showing off his unrestricted access to patients. The real Faith arrives and Sherlock discovers she is not his visitor. His violent reaction leads John to attack him, and Sherlock becomes a patient himself. Smith visits his room, admits his murders and begins suffocating him, believing nobody will intervene.
John returns after finding Mary's instructions that Sherlock put himself in danger so John would have to save him. He stops Smith, whose confession has been recorded inside John's old walking stick. The friends reconcile, and John admits his secret messages to another woman. At his next therapy session, the therapist reveals that she was both that correspondent and the woman posing as Faith. She introduces herself as Eurus, Sherlock and Mycroft's sister, then fires at John.