Season 4

Complete listed story — includes the ending

1. The Six Thatchers

Sherlock is asked to investigate the mysterious death of a young man, which he solves quickly but he is led into another mystery when a bust of Margaret Thatcher owned by the dead man's father is smashed. Further busts are smashed and Sherlock discovers that the mystery is linked to Mary and her past as a government agent. A figure from her past is bent on revenge in the belief that Mary betrayed him, but it is discovered that the traitor was the secretary of a British Parliamentary member. Mary is killed by the secretary when she jumps in front of a bullet meant for Sherlock. John blames Sherlock for Mary's death, and their relationship is fractured. The title and plot are loosely based on the Doyle short story " The Adventure of the Six Napoleons ".

2. The Lying Detective

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 ".

3. The Final Problem

Sherlock and Watson – who were shot with a tranquilliser by Eurus – trick Mycroft into acknowledging her existence. Eurus steps up her attacks on Sherlock, culminating in the bombing of his Baker Street apartment. Sherlock, Watson and Mycroft venture forth to Sherrinford, a maximum-security psychiatric facility where Eurus is housed. Although Mycroft is skeptical at the suggestion that she escaped, the trio discover that Eurus compromised the staff and controls the entire asylum. She subjects the trio to various ordeals, testing their morals by forcing them to choose which of her victims live and die and ultimately forcing Sherlock to confront the memory of "Redbeard", a childhood friend whose death set in motion events that saw Eurus incarcerated. Realising that she will continue to test him until someone he cares for dies, Sherlock tries to connect with her on an emotional level, offering the love and relationship with a brother that she never had, and Eurus stands down. Sherlock and Watson return to their apartment, and find another message from Mary imploring them to stay together. The two rebuild the flat to its original form before meeting various unusual clients. The title matches the Doyle short story " The Final Problem ", but the plot contains elements from the short stories " The Adventure of the Musgrave Ritual ", " The Adventure of the Three Garridebs ", and " The Adventure of the Gloria Scott ".