Season synopsis
Contains spoilers for this season and earlier seasons.
Two years after his apparent death, Sherlock returns to London following his work dismantling Moriarty’s network. John has begun a new life with Mary Morstan and reacts furiously to the discovery that his friend let him grieve. Their reunion becomes entangled with a terrorist plot: a vanished Underground carriage has been packed with explosives beneath Parliament. Sherlock and John find it and stop the attack, while Sherlock and Mary also rescue John from a bonfire after an unexplained kidnapping. The friendship resumes, but Mary is now central to John’s life.
Sherlock serves as best man at their wedding, struggling to turn his affection for John into a speech. Recounting earlier investigations helps him connect an apparently impossible stabbing to a threat against guest Major Sholto, John’s former commanding officer. The wedding photographer has targeted Sholto in revenge for his brother’s death under Sholto’s command; a tight uniform belt has concealed the wound. Sherlock gets Sholto to accept medical help, and the photographer is arrested. He also realises Mary is pregnant and promises to remain there for the couple.
Newspaper proprietor Charles Augustus Magnussen threatens that future through his blackmail operation. Investigating him, Sherlock finds Mary holding him at gunpoint; she shoots Sherlock, who survives and exposes her hidden life as an assassin to John. John eventually chooses reconciliation. Sherlock attempts to trade Mycroft’s laptop for Magnussen’s information on Mary, expecting authorities to seize a physical archive. Magnussen reveals that his secrets exist only in his memory. With the trap defeated, Sherlock kills him and faces a dangerous overseas assignment. His departure is abruptly cancelled when screens across Britain broadcast Moriarty’s image. The message suggests another threat without establishing that Moriarty has survived.