Sherlock faces one of the most chilling enemies of his long career: the powerful and seemingly unassailable Culverton Smith - a man with a very dark secret indeed.
Sherlock remains distant to John Watson, who is still grieving after the death of his wife, Mary. John, who is seeing a new therapist, keeps it to himself that he is hallucinating conversations with 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 afterward and writes it down on a sheet of paper. She comes to Sherlock, who has started using drugs again and is clearly suffering from the side effects, with the sheet of paper saying that there was one word—one name—that shook her. Sherlock dismisses her case as being too weird and refers her to Scotland Yard. Before she leaves, Sherlock realizes that Faith is suicidal, and offers to walk with her around London at night despite him being still high. 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, and he decides he will take on her case. Sherlock is momentarily overwhelmed by side effects of his drug use and upon calming himself, finds Faith gone and nowhere to be seen. As Sherlock makes his way back to his flat stumbling and disoriented, he realizes that the one word was "anyone".
Sherlock becomes inexplicably obsessed with Smith, but his crazy antics as a result from his drug use frighten Mrs. Hudson, who effectively subdues him. Mrs. Hudson races with an intoxicated and bound Sherlock in the boot of her Aston Martin sports car to see John while he is at his therapist's house for a session. Sherlock suspects Smith of being a serial killer and asks John to help him bring him down. Much to John's shock and annoyance, Mrs. Hudson's arrival with Sherlock , a car sent by Smith, and Molly Hooper with an ambulance, were all prearranged to arrive at his therapist's house at that exact time by Sherlock weeks prior before John had even chosen a new therapist.
Sherlock and John arrives at a studio to meet with Culverton Smith. As Smith takes them to visit the new hospital wing for which he was a major donor and thus also named after him, he subtly hints of being a serial killer to Sherlock and John while meeting with a group of children at the hospital. Smith continues to give them a tour of the hospital by taking Sherlock and John down to his favorite room, the mortuary. Sherlock tries to goad Smith into a confession just as Faith arrives—called by Sherlock via Smith's phone to come—but upon seeing her, realises that the woman that came to his flat was not actually Faith. Mentally unstable from drug abuse and frustrated, Sherlock makes a move to attack Smith with a scalpel, but John stops him and beats up Sherlock out of anger, blaming him for Mary's death.
Sherlock is admitted into Smith's hospital, and Smith visits him, coming in through a secret passageway. Sherlock states that he wishes for Smith to kill him despite not wanting to die and Smith complies after confessing to his murders. Unknown to him, Sherlock is acting on the orders of Mary who asked him earlier, through a recorded video message given to him posthumously, to save John by putting himself in harm's way so John could rescue him and thus mend their broken friendship. John, after seeing Mary's video himself at Sherlock's flat, rushes to the hospital to save Sherlock. He arrives just in time to pull Smith off from suffocating Sherlock. Believing he can get away with no proof of his confession, Smith is surprised when Sherlock reveals Smith's confession was recorded by a hidden device in John's old walking stick which John had left behind earlier.
John reconciles with Sherlock, telling him he no longer blames him for Mary's death and confesses to a hallucinated Mary that he had cheated on her, but only through text conversations with another woman. At the same time, he learns that Sherlock saved Irene Adler, and is trying to avoid having a relationship with her. John encourages him, even offering to have cake for his birthday. With his spirits revived, Sherlock dons the deerstalker, heading out to celebrate his birthday with John and Molly.
John, once again, goes to see his therapist, but the therapist reveals that she was the one who pretended to be both Faith and the woman John cheated with. She reveals her true identity to be Eurus (Greek for the east wind), purportedly Sherlock and Mycroft's secret sister. As John attempts to leave, she pulls out a gun, points it at John, and pulls the trigger.
- The title of the episode is a reference to the story "The Adventure of the Dying Detective".
- The villain, Culverton Smith, as well as the basic idea of his crimes and his confession to a dying Sherlock are also references from The Adventure of the Dying Detective.
- Sherlock's words to Fake Faith upon realizing she is planning to commit suicide– "Your life is not your own, keep your hands off it."– are taken from the story "The Adventure of the Veiled Lodger".
- Benedict Cumberbatch - Sherlock Holmes
- Martin Freeman - Dr John Watson
- Una Stubbs - Mrs Hudson
- Rupert Graves - Inspector Greg Lestrade
- Mark Gatiss – Mycroft Holmes
- Amanda Abbington – Mary Watson
- Louise Brealey – Molly Hooper
- Toby Jones - Culverton Smith
- Siân Brooke - Eurus Holmes
- Chris Wilson - High Rank Police Officer
"Taking your own life. Interesting expression– taking it from who? Once it's over, it's not you who'll miss it. Your own death is something that happens to everybody else. Your life is not your own. Keep your hands off it."
–Sherlock Holmes, to Faith Culverton
"I need a second opinion."
"Oh, please, John. Since when have you ever managed TWO opinions. You'd fall over."
"No. I need you examined by another doctor. Someone, who unlike me, learned to see through all your bullshit long ago."
"I can't possibly think of anyone like that."
Sherlock is silent
"Did you hear me? I want you to be examined by Molly Hooper."
"You're REALLY not got going to like what happens next."
―Sherlock knows John hates his surprises
"Once more onto the breech, dear friends! Once more! Or close the walls up with our English dead! Set the teeth and stretch the nostril wide. And build every spirit up to his full height."
―Sherlock, quoting Henry V (Act 3, Scene 1, 1-6) while madly running around the flat, preparing to take on Culverton Smith.
"I have been many things, but when have I ever been a malingerer?"
"You pretended to be dead for two years!"
"Apart from that."
―Sherlock and John argue
“Must be something comforting about the number three. People always give up after three…”
- "How is he?"
"FINE? Sherlock, if you keep taking what you're taking at the rate you're taking it, you have weeks at best."
"Exactly: weeks! Let's not get ahead of ourselves."
"It's not funny!"
"Tiny bit is. Are you okay, Molly, You seem very stressed."
"I'm stressed; you're dying."
"Then I'm ahead then. Dying can only ruin one day of your life; stress can ruin everyday.""
- ―John and Molly worry about Sherlock's health, while he is uncaring of his drug overusage
- "So, Molly's meeting us at this cake place?"
"It's your birthday; cake is obligatory."
"I suppose a sugar high is some kind of substitute."
- ―Sherlock and John discuss celebrating his birthday
- This is the second episode in the series not to have any appearance or mention of Jim Moriarty, with the first being "The Sign of Three".
- It is the second episode where Sherlock doesn't use drugs to help him to think with the first being His Last Vow.
- Sherlock shows his ability to deduce things becomes sloppy when he's been on drugs too long (in conjunction with his boredom); he was able to examine the page he got from Fake Faith within moments, but it took him longer to realize she was (apparently) suicidal.
- Sherlock knows the streets of London well enough to spell out vulgar messages to Mycroft. (Although the "F" in the message is cut off to allow it to be seen.
- Apparently, people have begun mistaking John's blog for Sherlock's as John always writes about their cases (despite the name "John H. Watson" obviously being on it).
- Adding to the humor; no-one really seems to know who John is, despite his brotherly relationship with Sherlock and the amount years they've spent solving crimes.
- To the shock of John, Mrs. Hudson and Molly, Sherlock is able to predict what they will do in the future based on what he knows about them.
- As John points out, Sherlock planned all of this two weeks in advance.
- It's noted that footage of Sherlock kept going in and out when he was walking with Fake Faith. When it's revealed to have been Eurus in disguise, it's likely she managed to hack into the surveillance system of the country and program it to go off when she walked into view.
- It's strange that Sherlock didn't bring up the cook who made the chips he and Eurus ate; that would have proven that she wasn't a a hallucination.
- According to Mrs. Hudson, Sherlock doesn't just shoot the wall of his flat when he's bored (The Great Game); he also shoot it in frustration when a case is unsolved. Another thing Sherlock does is karate chop the fridge when his breakfast isn't made. Something both she and John know is that Sherlock will stab something he can't figure out into the fireplace mantle.
- This heavily implies that Sherlock had spent a long time trying to figure out how to carry out Mary's last wish to mend his relationship with John. (She told Sherlock to pick a fight with a villain and lose, to bring John back; it's likely Sherlock was waiting for a worthy target.)
- Mycrfot notes that as a child, Sherlock set a trap for Santa Claus, noting it was the only other time he showed a kind of obsession.
- Sherlock quotes Shakespeare in Mrs. Hudson's flashback, even reaching the original Sherlock catchphrase "the game's afoot" at the end of his quotation.
- If John had not found the DVD of Mary's message to Sherlock, he wouldn't have gone to save him at the hospital. (Up until that point, he assumed Culverton Smith was innocent and accused by Sherlock due to being high.)
- Sherlock's birthday was known only to his family and Irene Adler at this point.
- As of this episode, John and Molly know as well.
- Arthur Conan Doyle was to some degree inspired by Edgar Allan Poe's Dupin-Stories. The murderous Orang Utan mentioned by Holmes hints at Poe's "The Murders in the Rue Morgue". Also there's a motif of things hidden in plain sight in this episode which is taken from another of Poe's Dupin-Story, "The Purloined Letter" in which a Dupin is hired to find a compromising letter which turns out to be hidden where no one would look for it - in plain sight.