[s4e1] The Six Thatchers -

The investigation reveals that the busts are connected not to Moriarty, but to Mary’s (Amanda Abbington) former life as a freelance assassin in a group called "A.G.R.A.". A former teammate, Ajay, believes Mary betrayed them years ago during a failed mission in Tbilisi and seeks revenge.

" The Six Thatchers ," the premiere episode of Sherlock Season 4, marks a significant shift in the series' tone, blending high-speed detective work with a tragic exploration of its characters' pasts. Loosely based on Sir Arthur Conan Doyle's "The Adventure of the Six Napoleons," the episode transitions from a series of lighthearted casework montages into a dark, spy-thriller narrative centered on the secret history of Mary Watson. [S4E1] The Six Thatchers

It explores the concept of "The Appointment in Samarra," emphasizing that no one can truly outrun their past. Sherlock S4E1 “The Six Thatchers” (BBC One) The investigation reveals that the busts are connected

In a dramatic confrontation at the London Aquarium, it is revealed that the true betrayer was Vivian Norbury, a high-ranking British intelligence official. When Norbury fires a shot at Sherlock, Mary jumps in front of him, sacrificing her life to save his. This tragedy leaves John Watson (Martin Freeman) devastated and deeply resentful of Sherlock, fracturing their partnership. Loosely based on Sir Arthur Conan Doyle's "The

The episode is noted for its sharp divide between the first half’s traditional detective fun and the second half’s heavy emotional stakes.