Sherlock discovers Mary's secret when he finds her at Magnussen's office, held at gunpoint. It is revealed that Mary was formerly a high-level assassin, and she shoots Sherlock to keep her past hidden from John. After a miraculous recovery in the hospital—assisted by a mental journey into his "Mind Palace"—Sherlock decides to protect the Watsons at any cost. The Showdown at Appledore
The story begins with John Watson finding Sherlock in a drug den, though Sherlock claims he is undercover to investigate Magnussen . Magnussen is a master blackmailer who holds leverage over everyone, including the highest members of the British government and, most shockingly, John’s new wife, . The Ultimate Betrayal
Detailed transcripts and further reading for this episode can be found on sites like the Ariane DeVere LiveJournal or IMDb .
To protect Mary and John from a lifetime of manipulation, Sherlock makes a shocking choice. In front of Mycroft and the police, he shoots Magnussen , committing a "vow" to keep his friends safe. The Final Twist
In the gripping finale of Season 3, titled Sherlock Holmes faces his most repulsive adversary yet: Charles Augustus Magnussen, the "Napoleon of Blackmail". The Web of Secrets
Sherlock is sentenced to a dangerous undercover mission abroad as punishment for the murder. However, as his plane takes off, every screen in Britain is hijacked by a familiar face. appears with a simple, chilling question: "Did you miss me?" This forced return ensures Sherlock's exile is cut short after only four minutes.