Contains spoilers for this episode and earlier episodes.
Sherlock faces John's wedding to Mary with a difficult assignment: deliver a best man's speech. His account of their friendship brings the guests into earlier cases, including a guardsman found bleeding inside a locked shower. Sherlock had concentrated on the impossible attack, while John noticed that the man was alive and saved him. Another apparently unsolved investigation concerns women who dated a man using the identities of the dead.
As Sherlock speaks, a detail connects those women to wedding guest Major Sholto, John's former commanding officer. Someone has courted members of Sholto's staff to discover his movements and reach him at the reception. Sherlock realises that the earlier stabbing was a rehearsal. A narrow blade passed through a tightly fastened uniform belt, which held the wound closed until the victim undressed. Sholto is already wounded without realising it.
Sholto locks himself away and considers allowing himself to die, but Sherlock persuades him to accept treatment rather than devastate John's wedding. The photographer, Jonathan Small, is exposed as the attacker, seeking revenge for his brother's death under Sholto's command. The celebration resumes, with Sherlock playing violin for the couple's first dance. He then tells John and Mary that the changes he has noticed suggest she is pregnant. Once they return to the dance floor, Sherlock leaves the reception alone.