Sherlock: 10 Biggest Plot Twists
4. Watson’s Therapist Is Eurus – The Lying Detective
‘The Lying Detective’ was the best episode in the fourth series, and though the whole adventure was well done, it was the final scene that really hit home.
Following his wife’s death in 'The Six Thatchers', Watson invests in some therapy sessions with a woman he believes to be a legitimate therapist. It is later revealed that said therapist is in fact Eurus Holmes – the younger sister of Mycroft and Sherlock.
Owing to her unparalleled intelligence and her gift for manipulation, Eurus is able to go in and out of Sherrinford (her high-security prison) as she pleases.
Moreover, during her time in the outside world, Eurus disguises herself as two other people in addition to Watson’s therapist. Indeed, she masquerades as the daughter of Culverton Smith in order to have a conversation with Sherlock, and she also takes on the persona of an attractive young woman in an attempt to seduce Watson in the opening episode of Series 4.
As such, the twist at the end of ‘The Lying Detective’ has several layers to it given Watson’s therapist is actually one of several personas undertaken by Eurus.