Sherlock: 10 Biggest Plot Twists
2. Mary Is An Assassin – His Last Vow
Mary Watson – though certainly an intelligent lady – was never a cause for suspicion in the first two episodes of Series 3.
After breaking into Magnussen’s office, Sherlock finds him kneeling on the ground with a gun pointed at his head. Recognising the smell of Claire de la Lune, Sherlock presumes the assassin is Lady Smallwood, a British MP who had previously been blackmailed by Magnussen.
However, when the assassin turns around, Sherlock is met by the face of Mary Watson. Such a reveal takes both Sherlock and the audience completely off guard, and the surprises only continue as the famous detective is then shot by Mary.
Whereas most twists are only relevant to the episode they occur in, the revelation that Mary is an assassin challenges everything the audience thought they knew about her from the previous two episodes.
Such a twist also creates a great deal of sympathy for Watson. After the grieve he had to endure for two years following Sherlock’s fake suicide, the unveiling of his wife’s true identity is yet another emotional blow for him to take.