Sherlock S4 E3

A panic-stricken little girl wakes up on an aeroplane and finds everybody asleep. She picks a mobile phone up and hears Jim Moriarty announce "Welcome to the final problem".

Mycroft Holmes is at home, where Sherlock and Watson disable his home security to trick him into revealing that his sister, Eurus, exists. At 221B Baker Street, Mycroft explains that Eurus was an era-defining genius on a par with Isaac Newton, with abilities far greater than Sherlock's and Mycroft's, coupled with a total lack of normal sensation and emotion. Mycroft reveals that their parents sent Eurus to a mental institution after she kidnapped and drowned Sherlock's dog Redbeard, and after she burned their home down. Mycroft withheld this from Sherlock because Eurus had psychologically traumatized him as a child, taunting him with an enigmatic song, and leading ultimately to Mycroft having his memories 'rewritten', he'd also been lying to their parents about Eurus not surviving the blaze she had set. An adult Mycroft then sent her to a Government 'Black site' and maximum-security facility a quote "Prison within a prison", in the North Sea, Sherrinford, unknown to anyone outside very high ranking government officials and only on a need to know basis. When Mycroft insists that she is secured, a quadcopter carrying a motion-activated grenade flies into Sherlock's flat. John, Sherlock, and Mycroft flee as the bomb detonates, blasting Holmes and Watson out of the windows.

Later, John and Sherlock hijack a fishing trawler to travel to Sherrinford, carrying out a diversionary plan so that Sherlock can reach Eurus' cell. Mycroft and John corner the prison governor, discovering that he has explicitly disobeyed Mycroft's protocol and has allowed Eurus to interact with prison staff. Using her skill to "reprogram" everyone she speaks with, Eurus has effectively taken full control of the prison. Meanwhile, Sherlock talks to Eurus, but she attacks him and knocks him unconscious. The guards lock Sherlock, John, Mycroft, and the governor together in Eurus's old cell.

Mycroft reveals that five years before, he granted Eurus an unsupervised interview with Moriarty as a Christmas present in exchange for detecting national security threats to Britain. During that time, Moriarty agreed to record video messages for her. After forcing the governor to commit suicide, Eurus torments Sherlock, Mycroft and Watson in a series of psychological ordeals, forcing Sherlock into sinister games to save their lives while videos of Moriarty heckle him. Although Eurus forces Sherlock onward with the prospect of saving the girl on the aeroplane, he eventually stops the games by threatening to shoot himself when she orders him to shoot either John or Mycroft. Furious, Eurus uses tranquilliser darts on the three of them.

Sherlock wakes up near the burnt-out wreckage of his family's old house. He speaks to the girl in the aeroplane to try to guide her in landing safely. John wakes up chained at the bottom of a well. As Eurus raises the water level in the well, John finds a human skull there, and Sherlock realizes that what he thought was his dog Redbeard was in fact his childhood friend, Victor Trevor. Eurus threw him into the well and left him for dead because she felt left out of Sherlock's attention as a child. Sherlock then deciphers the real meaning of the song that Eurus originally taunted him with when Victor went missing, which reveals that she wants him to find her. Sherlock deduces that the girl in the plane is actually Eurus's mental metaphor and that the game has been a cry for help. With Eurus' puzzle solved, Sherlock is able to send her back to Sherrinford after rescuing John.

Mycroft explains to his and Sherlock's parents, who are angry that they had been told that Eurus was dead, that she refuses to speak to people anymore. Sherlock visits her and they play the violin together, giving performances for their parents and Mycroft. While helping Sherlock repair his destroyed flat, John receives a video sent by Mary before she died, encouraging him to continue working with Sherlock.