Sherlock S3 E2

In the opening scene, DI Lestrade and Donovan are on the verge of arresting the criminal Waters family that has evaded the police several times. However, when Lestrade receives a text for help from Sherlock he abandons the case and races to Baker Street, assuming the worst and calling for maximum backup – only to discover that Sherlock is simply struggling to write a best man speech for John's upcoming wedding to Mary Morstan

On the morning of the wedding, Mrs. Hudson (Una Stubbs) reminds Sherlock that marriage changes people. At the reception, John is delighted to see Major James Sholto (Alistair Petrie), his former Army CO. Sholto (the name is a reference to a character in The Sign of Four) lives in seclusion, having received death threats and media scrutiny after losing a unit of new soldiers in Afghanishab. Sherlock calls Mycroft, who repeats Mrs Hudson's suggestion that John and Mary's marriage will change his life.

Sherlock rises to give the best man speech, but he initially hesitates. After reading from the wedding telegrams, Sherlock expresses his deep love and respect for John and launches into a rambling narrative, describing John's role in an attempted murder case, "the Bloody Guardsman";  named Bainbridge contacted Sherlock, fearing he was being stalked. By the time Sherlock and Watson got into the Guards' quarters, Bainbridge was presumed dead in a shower room from a stab wound, but no weapon and escape route were found. When questioned by Lestrade, Sherlock reluctantly admits the case wasn't solved, but cites it as an example of John's compassion; instead of trying to solve the murder as Sherlock did, John examined Bainbridge's body and discovered he still had a pulse, thus requesting an ambulance and saving his life.

Sherlock's narrative drifts to another case, "the Mayfly Man"; several days after going to a man's apartment for dinner, Tessa a woman who worked as a private nurse, found the apartment was vacated, and the man died weeks ago. Sherlock and John, still from John's stag night attempted to search for clues, but were arrested for their drunken antics. The next morning, an amused Lestrade secured their release from jail. Sherlock chatted to other London women with a similar experience, but failed to find any significant connection between them. With John's help, he concluded the perpetrator was a man bored with marriage, who disguised himself as recently deceased single men and used their unoccupied homes to meet the women.

While moving to the toast, Sherlock suddenly freezes, recalling Tessa knew John's middle name. Aware that John hates and never uses it, he deduces Tessa saw it in a wedding invitation. Sherlock concludes all the women worked for Sholto in various capacities and were bound by confidentiality. The Mayfly Man courted them to find and attack Sholto, and the wedding is his chance. Sherlock slips a note to Sholto, who returns to his hotel room and gets his pistol to defend himself. Sherlock, John, and Mary race to the room to try and save him, but he refuses to open the door until the case is solved. Sherlock deduces the Bloody Guardsman case is linked to Sholto's, and pinpoints the military uniform both wore as the common link; since Bainbridge collapsed in the shower, he must have been stabbed with a type blade beforehand, but with his military waist belt firmly holding the flesh together, the damage would not take effect until the belt loosened and the victim would not feel it until then. Upon hearing this explanation, Sholto contemplates suicide by loosening his belt and bleeding to death. Sherlock persuades him not to, primarily by insisting that it would be cruel to do at John's wedding. Sholto then opens the door and requests medical assistance.

That evening, Sherlock apprehends the wedding and identifies him to Lestrade as Jonathan Small, the Mayfly Man, deducing he was the only person who could have stabbed Sholto. He points out the photographer's brother was one of the men killed under Sholto's command, and concludes that he stabbed Bainbridge as practice for this murder. After Sherlock plays the violin for John and Mary's first dance, he quietly reveals to them that he has observed in Mary "increased appetite, change in taste perception, and sickness in the morning, the signs of three", revealing she is pregnant. Sherlock calms them by explaining that they will make great parents, since they've had plenty of practice with him. Despite the happy revelation, the episode ends on a bitter-sweet note, with Sherlock sombrely leaving the reception alone, upon realizing that his relationship with John will never be the same again