- "Hello, my name is Jim Moriarty. Welcome to the final problem."
- ―Jim Moriarty in a pre-recorded message
In the final episode of this series, written by Mark Gatiss and Steven Moffat, long-buried secrets finally catch up with the Baker Street duo. Someone has been playing a very long game indeed and, alone and defenseless, Sherlock and Dr. Watson face their greatest ever challenge. Is the game finally over?
A young girl wakes up on an aeroplane and is deeply troubled that her mother sitting beside her and everyone on the plane, including the pilots are unconscious, the oxygen masks deployed. A passenger's mobile phone rings and when she answers it pleading for help, she hears a man tell her, "Hello, my name is Jim Moriarty. Welcome to the final problem."
Mycroft is enjoying a vintage film in the private cinema in his home. After a while, the film footage is disrupted by footage from his childhood before ominously flashing the message, "I'M BACK" right before the reel ends. Visibly bothered by this, Mycroft tries to leave but finds the main door locked. He is forced to go through a hidden door that mysteriously opens and slams behind him. Taking a sword disguised as an umbrella to defend himself, he follows a girl running about and whispers coming from his walls to an upstairs corridor. Around a corner, a clown confronts him, arms himself with a nearby sword, and poises himself to challenge Mycroft to a sword battle. Mycroft removes the blade of his sword to reveal a hidden gun, but it fails to shoot. The clown cries out and lunges at Mycroft who dodges him and flees downstairs. He tries to open all the doors to escape finding them all also locked. As panic sets in for Mycroft, Sherlock and John appear and reveal the whole situation to be a ploy to prove the existence of the Holmes brothers' secret sister, Eurus Holmes, as well as the fact that she had been incarcerated at a facility controlled by Mycroft from an early age. John tells Mycroft he thought the latter would never admit the truth unless put into a terrifying situation.
Back at Baker Street, Mycroft comes to meet with Sherlock and John and is told to sit down like a regular client if he wants to talk. Reluctantly, he does so, after which Sherlock demands that Mycroft tell him the whole truth, as he cannot remember Eurus at all, or indeed having ever had another sibling. Mycroft describes Eurus as being more brilliant than either of the Holmes brothers, which was confirmed in professional assessments done in their childhood. Even though Sherlock has no memory of Eurus, Mycroft tells him that every action and decision that defines Sherlock is based on Sherlock's memory of Eurus. Mycroft retells them disturbing instances from Eurus' childhood at their family ancestral home, Musgrave, such as cutting herself to see how her muscles worked or cryptically singing about how and where she killed their dog, Redbeard. After she sets fire and burns down their home, she was taken away. Mycroft reveals that their parents were told the lie that she died there after starting another fire. In reality, he admits, she had been contained at a maximum security facility called Sherrinford ever since. Just as Mycroft dismisses the idea that she has escaped, they hear Eurus singing her cryptic song as a drone flies into the flat carrying a powerful bomb rendering them to be still since it will detonate when sensing motion. They make a plan of escape and then move, setting off the bomb just as John and Sherlock jump out the windows and Mycroft makes for the doorway.
At sea, Sherlock and John are dropped by helicopter onto an unsuspecting boat and pirate it to travel to Sherrinford. It is revealed that Mycroft was severely injured by the explosion and is in the hospital unconscious with little hope of recovery. The boat captain and his second mate are found on a nearby beach of the island with a message written in the sand regarding Eurus. A fake-looking captain and John are found on another taken into custody for questioning. The head of security believes that he sees through the fake-looking captain's disguise and has captured John and Sherlock. He gives a security card to the guard to check on Eurus. The captain removes the prosthetics and reveals himself to actually be Mycroft and that the man who received the security card was in fact Sherlock. Sherlock sneaks past security and is able to visit his sister for the first time since their childhood. After a short conversation which leaves Sherlock visibly nonplussed with him asking Eurus repeatedly how she escaped, she coaxes Sherlock to step closer to the glass of her prison so she can tell him the truth of how she escaped. As he attempts to touch the glass, he instead touches her hand behind it and realizes that there was never any glass there.
Meanwhile, Mycroft, John and the governor review tapes of Eurus's psychiatric sessions, all of which happened against Mycroft's express instructions; he forbade any kind of communication with Eurus because of her powerful manipulative abilities. The governor mentions that whoever came into contact with Eurus was invariably corrupted and put under her influence, recalling an instance in which she talked a member of the Sherrinford staff into killing himself and his family. John then realizes the other voice on the tape they were currently watching was the governor himself and he, too, was under the influence of Eurus. Alarmed at realizing the security of the compound was already compromised, Mycroft and John attempt to leave. The governor apologizes and sounds the alarm, with guards capturing them to take them away. John hears the security alarm and realizes it is Moriarty talking gibberish, just as security screens flash his face and say, "Did you miss me?", before John is knocked unconscious.
In a flashback from Christmas five years prior, a helicopter arrives at Sherrinford with Moriarty onboard by Mycroft's orders. At Eurus' request for a Christmas present, she wanted five minutes of unsupervised time with Moriarty. According to Mycroft, Eurus had taken an interest in Moriarty's obsession with Sherlock, and she had him record the audio and video that the girl heard on the phone and John saw before being knocked unconscious, as well as many hundreds of other messages.
John awakes in a Sherrinford prison cell along with Sherlock, Mycroft, and the head of security. They briefly talk to the girl on the aeroplane before the connection is terminated by Eurus, who communicates with them via television screens around the compound. She gives them a choice in order to reconnect with the girl: Sherlock must choose either John or Mycroft to kill the head of security, or she will kill the man's wife whom she has taken captive. Mycroft declines the gun from Sherlock but John accepts it. Nonetheless, John cannot bring himself to shoot, so the head of security takes the gun from John and shoots himself before anyone can react. Because neither John nor Mycroft actually carried out the deed and her demands were therefore not met, Eurus shoots the wife anyway while remarking on her fascination with the moral standards the men hold themselves to. She tells Sherlock to take the gun with one bullet left because he will need it later.
She opens a secret cell door leading to a series of rooms with puzzles she has created for them. In the first room, Eurus has left them an envelope with images of suspects of a crime in which a man was shot, as well as the rifle he was shot with. She demands they solve who did it in exchange for more time to talk with the girl on the plane. To further pressure Sherlock emotionally, she suspends the three suspects outside the window bound and hanging over the sea, asking Sherlock to condemn one of them directly. Upon choosing one of the suspects, Eurus unexpectedly lets the other two men fall to their deaths instead, and after John reacts angrily to this pointless murder, she lets the condemned fall as well, remarking that there really is no difference between guilty or innocent.
Eurus leads them into the next room, where an empty coffin sits with the lid simply saying, "I LOVE YOU". After some time, Sherlock and John deduce that the coffin is intended for Molly Hooper. Eurus explains that she has rigged Molly's apartment with explosives, and in order to call off the detonation Sherlock must get Molly to say "I love you" on the phone within a time limit of three minutes, on the condition that he cannot reveal in any way that there is a threat or danger involved. Sherlock is thereby forced to make Molly confess her love for him, which leaves her emotionally shaken. Afterwards, Eurus reveals there were never any bombs and Sherlock had in fact lost rather than won, since the only result was Molly now having been emotionally hurt by him. Before leaving the room, Sherlock puts the cover on the coffin and suddenly breaks down, punching and destroying the coffin while yelling in frustration.
The next room is empty. Eurus tells Sherlock he now has to choose either John or Mycroft to continue on with him, and to kill the other. Mycroft proclaims that intellect rather than sentiment is what is needed going forward, and that the choice should be obvious: John. John tries to argue but after a moment quietly remarks that he believes Mycroft is, in fact, right. Sherlock, clearly unable to kill John, is taunted by Mycroft. This eventually leads Sherlock to point the gun at him instead, but after realizing it took only five minutes for Moriarty and Eurus to devise the entire plan that pitted them against one another, Sherlock in a desperate move puts the gun to his own throat while threatening Eurus with a ten-second countdown. Not getting the result she wanted, Eurus shoots them with a tranquiliser dart from the walls.
Sherlock wakes up hours later in a cell with images scattered on the walls. He is reconnected with the girl and starts to guide her in landing the plane before being cut to John who has woken up chained at the bottom of a well. Realising the cell is a ruse, Sherlock easily pushes down the wall to see that he is at Musgrave. Eurus once again challenges him to solve his first case, the Musgrave Ritual, and save John. Sherlock simultaneously guides the girl in landing the plane and solves Euros' puzzle while Eurus mocks him with her cryptic song. Already having analysed her song's lyrics with no results, Eurus reminds him that Redbeard was not a dog—for they could never have one because of their father's allergy to them—but rather the nickname for his best friend, Victor Trevor. This revelation comes as John figures out the bones in the well are human, not canine. Sherlock remembered Redbeard as a dog to cope with being upset with the horrific truth of his friend's death. Sherlock deciphers the cryptic song and realises that it along with the girl on the plane (which was a mental metaphor) was Eurus asking Sherlock for help. Finally getting the love she was denied in childhood from Sherlock, she discloses where John is and he is rescued. It is revealed Mycroft was safe and had only been locked away in her old cell in Sherrinford.
Eurus is taken back into custody at Sherrinford since she had killed people during her trials for Sherlock. Mycroft admits to their parents that Eurus is alive, but cannot see her since she will talk to no one. Mr and Ms Holmes are furious with Mycroft for keeping the truth from them and eventually ask Sherlock what to do next, surprisingly remarking he "was always the grown-up one", which startles Mycroft. Sherlock can be seen visiting Eurus regularly and although they do not talk, they play violin duets together separated only by the now-solid glass pane of her cell. One time, we also see Mycroft and Mr. and Mrs. Holmes sitting in on one of Sherlock and Eurus' violin duets.
Sherlock and John return home to clean up and repair their Baker Street flat. John receives an anonymous CD with "MISS YOU" written on it which he watches with Sherlock. It is another message from his late wife, Mary Morstan, who encourages him to continue working with Sherlock in solving cases and having adventures together.
- When Mycroft is explaining things in his office to Mr. and Mrs. Holmes, Sherlock is shown leaning on the door in the exact same manner as he does on the back cover for Season 4.
- The "dancing man" code is seen on a blackboard during the montage at the end.
- This episode shows that while Mycroft is smarter than Sherlock when it comes to impartial logic, Sherlock is actually more emotionally mature than him.
- Unlike the other season ends, there is no cliffhanger. (Thus if it was decided to no longer continue the show, the ending would be acceptable.)
- It's revealed that Sherlock is the least intelligent of his siblings; Mycroft may be smarter than him, but Eurus is actually the smartest of the siblings, despite her younger age.
- Given everyone in Sherrinford was obeying Eurus, it's likely that once she gained control of the governor, she had him send in the staff one-by-one until she reprogrammed the entire institution.
- Apparently, Sherlock plays the violin poorly. Eurus tells him he doesn't "understand" Bach, just as Moriarty had said that Bach himself would be appalled by Sherlock's playing.
- Eurus' Musgrave Ritual song has the same melody as "Rousseau's Dream" by Jean-Jacques Rousseau, which is also known as "Go Tell Aunt Rhody" in America and is very popular in Japan as a melody for children's songs. Its origins are believed to date back to the nineteenth century.
- When Lestrade calls Sherlock a "good man" is and echo back to the pilot, "A Study in Pink" where he tells John that the reason he puts up with Sherlock is that "Sherlock Holmes is a great man. and if we are really lucky, one day, he might be a good one." This is meant to show how just how much Sherlock has changed.
- "Experiment complete. Conclusion: I have a sister."
"This was YOU?"
"Conclusion 2: My sister, Eurus apparently, has been institutionalized from an early age by my brother. (Smiles and waves) Hey, bro."
"Why would you do this?"
"Conclusion 3: You are TERRIFIED of her."
"You have no idea what you're dealing with."
"New information: she's out."
"That's not possible."
"More than possible. She was John's therapist."
"Still had ten minutes to go."
"We'll see about a refund.""
- ―Sherlock and John discuss Eurus after scaring Mycroft
- "Is that vibrato, or is your hand shaking?"
- ―Eurus, about Sherlock's playing after she tells him something disturbing
- "Victor Trevor. We played pirates together. I was Yellowbeard, and he... was.... Redbeard. You killed him. You killed my best friend."
"I never had a best friend."
- ―Sherlock and Eurus discuss the past
- "Dr. Watson? Could you please leave."
"This is about family."
"THAT'S why he stays!"
- ―In a brief moment of anger towards Mycroft, Sherlock reveals he considers John family
- "The memories are disturbing."
"What do you mean? Examples."
"Mother and Father caught her with a knife once. She appeared to be cutting herself. They thought it was a suicide attempt. But when I asked Eurus what she was doing she said..."
"I wanted to see how my muscles worked."
"So I asked her if she felt pain. And she said..."
"Which one's pain? "
- ―Mycroft and his mental image of Eurus explain what the real one was like to Sherlock and John
- "That's the problem with name-badges. Everybody stops looking at faces. We'd be safer off with clown suits."
- ―Mycroft notes the incompetence of Sherriford's security
- "Good and bad are fairy tales. We have evolved to place an emotional value on what is nothing more than the survival instinct of the pact animal.Good isn't good. Bad isn't bad. Bottoms aren't really pretty. You are a prisoner of your own flesh."
"Why aren't you?"
"I'm far too brilliant."
- ―Eurus' interview tape
- "That's Sherlock Holmes is it?"
"You've heard of him?"
"Of course, sir. He's a great man."
"No. He's better than that. He's a good one.""
- ―Lestrade discuss Sherlock with an officer, seeing his wish for Sherlock came true
- "There's a flat on Baker Street, where two men sit & argue; like they've always been there and they always will. The best and wisest men I've ever known. My Baker Street boys: Sherlock Holmes & Dr. Watson."
- ―Mary's final message
- The title of the episode is a reference to the story "The Adventure of the Final Problem".
- Eurus makes reference to Jim Moriarty having a brother who is a station master, which is taken from The Valley of Fear. Also, a brother by the name of Colonel James Moriarty is mentioned by John Watson in The Adventure of the Final Problem.
- Mary's closing line referring to Sherlock and John as the best and the wisest men I have ever known is taken from the ending of The Adventures of the Final Problem in which John refers to the (then presumed deceased) Sherlock as The best, and wisest man whom I have ever known... The closing line from The Adventures of the Final Problem was also said by John in "The Empty Hearse"; John said the exact same line, adding that he forgave Sherlock for missing for two years.
- The Sherrinford prison is a reference to the proposed elder brother of Sherlock and Mycroft, Sherrinford Holmes.
- The Holmes's ancestral home (Musgrave) is an allusion to the story "The Adventure of the Musgrave Ritual".
- The men drawn on the chalkboard during the final montage is a reference to "The Adventure of the Dancing Men "
|Sherlock Holmes (Benedict Cumberbatch) • John Watson (Martin Freeman) • Mycroft Holmes (Mark Gatiss) • Jim Moriarty (Andrew Scott) • Greg Lestrade (Rupert Graves) • Molly Hooper (Louise Brealey) • Mrs Hudson (Una Stubbs)|
|Irene Adler • Mary Watson • Charles Augustus Magnussen • Eurus Holmes|
|"A Study in Pink" • "The Blind Banker" • "The Great Game"|
|"A Scandal in Belgravia" • "The Hounds of Baskerville" • "The Reichenbach Fall"|
|"The Empty Hearse" • "The Sign of Three" • "His Last Vow"|
|"The Six Thatchers" • "The Lying Detective" • "The Final Problem"|
|Unaired Pilot • "Many Happy Returns" • "The Abominable Bride"|
|"Sherlock: The Network" • Manga|