Mary Watson didn’t make it past Sherlock’s season 4 premiere, but why was she killed? The Great Detective has gone through a variety of adaptations since his debut in A Study in Scarlet in 1887, exploring realms like television, film, video games, comic books, and even other literary works from authors that have given their own spin to Sherlock Holmes, John Watson, and other characters created by Arthur Conan Doyle.
Among the most recent adaptations is the BBC’s Sherlock. Created by Stephen Moffat and Mark Gatiss, Sherlock brought the famous detective to the modern world, adapting some of his most famous cases to the present day and its technological advances. Sherlock also welcomed some of Doyle’s most popular (and important) characters from the detective’s stories, such as Irene Adler (Lara Pulver), Jim Moriarty (Andrew Scott), and Mary Morstan (Amanda Abbington), later known as Mary Watson after marrying John Watson (Martin Freeman). Mary made her first appearance in season 3’s “The Empty Hearse”, and she was very different to the Mary fans know from the books.
Sherlock’s Mary Watson had a dark past that Sherlock ended up uncovering. Her real name was Rosamund Mary, and she was a trained assassin who went rogue and changed her identity for her own protection. Her identity was exposed in season 3’s finale “His Last Vow”, where she tried to kill Sherlock after being blackmailed by Charles Augusts Magnussen (Lars Mikkelsen). Mary returned in season 4’s premiere episode “The Six Thatchers”, where her past continued to haunt her. With the organization she used to work for going after her, Sherlock confronted Lady Smallwood's secretary, Vivian Norbury, who gave the order in the mission that killed half of the organization, with Mary being one of the few survivors. Norbury shot Sherlock, but Mary jumped in the way and took the bullet, dying on the spot but not before professing her love and gratitude for John and Sherlock.
Mary Watson’s death affected the relationship between John and Sherlock, as the former blamed the latter for it. Mary continued appearing either as a hallucination or in video, and served as a push for John to reconcile with Sherlock and continue working with him. In the source material, Mary also dies, though how and exactly when is unknown. Now, there aren’t behind-the-scenes reasons for Amanda Abbington to leave Sherlock, unlike other shows where actors leave for different reasons, so her departure was all because the character’s time in the series had come to an end.
Mary’s death was important to keep John’s arc moving in season 4, with Moffat explaining to EW back in 2017 that she had to die because “Sherlock Holmes is about Sherlock and Dr. Watson and it’s always going to come back to that”, and as fun as it was to have the three of them together, the plan was always for Mary to go so the series could go back to just the two men. Arguably, the character of Mary Watson could have been handled better in Sherlock, as her criminal backstory felt messy and unnecessary, but at least that part of John Watson’s life wasn’t left behind.