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"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^[1][2] on 8 January 2017.

Sherlock remains estranged from Watson, who is still grieving over the death of his wife, Mary. John, who is seeing a new therapist, conceals the fact that he is having hallucinations of Mary. Culverton Smith, a prominent entrepreneur and philanthropist, gathers close colleagues, including his daughter Faith, to confess that he needs to kill somebody. Before he does so, he forcibly injects them with a serum that inhibits memory. Faith is still able to remember fragments and writes them down on a sheet of paper. Three years later she comes to Sherlock, who has started abusing drugs again, and says that she remembers her father wanted to kill someone, and that a one-word name, which she cannot remember, changed her life. Sherlock dismisses her case as being too weird and tells her go to Scotland Yard. Before Faith leaves, Sherlock realises that she is suicidal and has a gun in her handbag; he offers to walk with her around London.

Notified that Sherlock has left his flat, Mycroft tracks his movement across the city, and calls John out of concern, inadvertently implying existence of another Holmes sibling. Sherlock and Faith walk all night, and the following morning he decides to take her case because of the impossibility that the name upon which her life turns consists of one word. He takes the gun from her and is momentarily overwhelmed by the side effects of his drug use. Upon gathering himself, he finds Faith gone. Stumbling back to his flat, Sherlock realises that the 'one word' was "anyone". Smith is a serial killer with the wealth and power to hide his crimes completely.

Sherlock becomes obsessed with Smith and publicly accuses him of being a serial killer. His drug-addled antics frighten Mrs. Hudson, who brings him – at gunpoint – to see John while he is at his therapist's house for a session. Much to John's shock and annoyance, a car arrives sent by Smith, prearranged by Sherlock two weeks prior, even before John had chosen his new therapist. Sherlock tells John he needs his help to stop Smith, whom he calls "the most despicable human being that I have ever encountered". Just as John expresses the need for a second opinion to evaluate Sherlock's condition, Molly Hooper arrives with an ambulance, also prearranged by Sherlock.

Sherlock and John arrive at a studio to meet with Smith, who claims that Sherlock's accusation was actually a publicity stunt. He is making a commercial to publicise his new brand of cereal, calling himself a "Cereal Killer". Smith takes them to visit a new hospital wing, named after him as a major donor. Smith takes Sherlock and John to his 'favourite room', whilst repeatedly referring to the serial killer H. H. Holmes. John asks Smith how he moves through all of the rooms freely, to which Smith replies that he has keys to the hospital.

Sherlock tries to goad Smith into a confession just as Faith arrives, having been texted by Sherlock from Smith's phone. Upon seeing her, Sherlock realises that she was not the woman who had come to his flat. Confused, frustrated and suffering from withdrawal, Sherlock attacks Smith with a scalpel, but is stopped by John. John attacks Sherlock out of anger, and a beaten Sherlock admits his responsibility for Mary's death.

Sherlock is admitted into Smith's hospital, where John pays him what he plans to be a last visit. Smith enters Sherlock's room through a secret door. Sherlock asks Smith to kill him by altering the dosage of intravenous drugs. As Smith prepares to make the adjustments, he goads Sherlock into admitting he doesn't want to die and admits killing people because he it gives him pleasure.

When John arrives at Baker Street, Mycroft is there with a team going through the remains of Sherlock's drugs binge. When Mycroft tells Mrs Hudson that he is trying to find what set off Sherlock's "programme of self-destruction," she breaks out in laughter and directs them to Mary's DVD, After watching Mary's video, John rushes to the hospital and arrives in time to pull Smith off Sherlock.

When Smith says his people found and removed three listening devices from Sherlock's coat, Sherlock reveals a fourth recording device hidden in the walking stick that John had left behind earlier. The events were part of Sherlock's elaborate plot to expose Smith and fulfill Mary's last wish for Sherlock to "save John" by "going to Hell".

John reconciles with Sherlock, telling him that he no longer blames him for Mary's death. He also confesses to imaginary Mary and Sherlock that he had cheated on Mary by texting with another woman. Mary is fine with this but Sherlock comforts John as he weeps over his confession, and finally John's hallucinations of Mary disappear.

Sherlock is intrigued by how his "hallucination" of Smith's daughter Faith had given him so much valuable information, but when he discovers the note she had left behind, he knows the woman was real. Upon inspecting the note under a black light Sherlock finds the message "Miss me?".

During John's next session, his therapist reveals she was the one who had pretended to be Faith – having received the real Faith's note from Smith – and the woman with whom John had been texting. Holding John at gunpoint, she reveals herself to be Eurus, Sherlock and Mycroft's secret sister. As John attempts to leave, Eurus pulls the trigger.

The episode was based loosely on the Doyle short story "The Adventure of the Dying Detective".^[3] The villain, Culverton Smith, was loosely based on disgraced British entertainer and charity fundraiser Jimmy Savile, who was a BBC icon.^[4] Sam Wolfson of The Guardian noted that the Savile-esque character must have caused consternation behind the scenes at the BBC. Wolfson gave "a respectful nod to the BBC," writing, "There must have been some unease in the corporation about having an episode in their flagship drama series in which a beloved public figure... uses his position of power and fame to commit monstrous crimes."^[5]

The episode received positive reviews from critics. Kaite Welsh of IndieWire graded "The Lying Detective" an A+, particularly the plotline of Sherlock descending into drug addiction: "He really does turn his kitchen into a meth lab, he is weeks away from death and hallucinating. It's the flip side of the genius that carries the show."^[4]

Vox rated the episode 3.5/5 stars.^[6] Sean O'Grady of The Independent gave the episode 4/5 stars, describing it as "Rarely can drama have come so morbidly loaded as last night's Sherlock, and rarely carry so much morbid fascination for the viewer".^[7] IGN gave the episode a good rating with a 7/10, describing it as "hampered by another unsatisfying case and some odd story beats".^[8]

Allison Shoemaker of The A.V. Club praised the episode and the story, grading it a B+, writing, "'The Lying Detective' does what it does so dazzlingly well that it's easy to overlook its misses... It's got a couple of good twists, some familiar to readers of the stories and others not. It's affecting and funny and moving and smart, and in short, a great script. But just because something checks all the right boxes doesn't mean it totally works, and Steven Moffat's script falls a bit flat where it really counts. It's as if he's blinded to the big picture by his own considerable gifts. This is a great Sherlock/Watson story with time to spare for everything but them."^[9]

• "The Lying Detective" at IMDb