Note: there will be spoilers in this discussion!
Synopsis: Sherlock is asked to investigate the mysterious death of a young man, which he solves quickly but he is led into another mystery when a bust of Margaret Thatcher owned by the dead man's father is smashed. Further busts are smashed and Sherlock discovers that the mystery is linked to Mary and her past as a government agent. A figure from her past is bent on revenge in the belief that Mary betrayed him, but it is discovered that the traitor was the secretary of a British Parliamentary member. Mary is killed by the secretary when jumping in front of a bullet meant for Sherlock. John blames Sherlock for Mary's death, and their relationship is fractured.
Aired on: January 1, 2017