"The Lying Detective" is the second episode of the fourth series of the British television series Sherlock and the twelfth episode overall. The episode was first broadcast on BBC One, BBC First, PBS and Channel One on 8 January 2017.

Sherlock remains distant to John Watson, who is still grieving the death of his wife Mary ("The Six Thatchers"). John, who is seeing a new therapist, keeps it to himself that he is having hallucinations of Mary. Culverton Smith, a prominent entrepreneur and philanthropist, gathers close colleagues, including his daughter, Faith, to confess he is going to kill somebody. But before he does so, he forcibly subjects them to a medical serum that inhibits memory causing them to not remember Smith's confession. Despite that, Faith is able to remember fragments shortly afterwards and writes it down on a sheet of paper. She comes to Sherlock, who has started abusing drugs again to the point of suffering from the side effects, with the sheet of paper saying that there was a name – one word – that shook her. Sherlock dismisses her case as being too weird and refers her to Scotland Yard. Before she leaves, Sherlock realises that Faith is suicidal, and offers to walk with her around London at night. Meanwhile, Mycroft tracks Sherlock's unusual movement across the city, even calling John out of concern, and accidentally slips an implication of another Holmes sibling. Sherlock and Faith walk all night, and the following morning he decides to take on her case. Sherlock is momentarily overwhelmed by the side effects of his drug abuse and upon gathering himself, finds Faith gone. As Sherlock stumbles his way back to his flat, he realises that the 'one name' was "anyone".

Sherlock becomes inexplicably obsessed with Smith, but his drug-addled antics frighten Mrs. Hudson, who effectively subdues him. Mrs. Hudson races with an intoxicated and bound Sherlock in the boot of her Aston Martin Vantage S, to see John while he is at his therapist's house for a session. Sherlock accuses Smith of being a serial killer and asks John to help bring him down. Much to John's shock and annoyance, the arrival of a car sent by Smith, and of Molly Hooper with an ambulance to his therapist's house at the exact time, were all prearranged by Sherlock two weeks prior, even before John had even chosen his new therapist.

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