Several high-ranking Government officials are questioning Charles Augustus Magnussen about his frequent meetings with the Prime Minister. He later meets Lady Elizabeth Smallwood, one of the officials, and blackmails her using information about her husband's past. She reluctantly obliges, and on her way home, decides to seek the services of Sherlock Holmes.
John Watson wakes up in the morning beside his wife, Mary. A distressed neighbour comes to them to ask for help regarding her drug-addicted son, who did not return home the previous night. John and Mary drive to the drug-den, with John finding the son, and surprisingly, also an undercover Sherlock. John is mad at Sherlock for abusing drugs (which Sherlock denies doing and insists that he is working incognito on a case) and not contacting him for a month after the wedding. They go to Bart's, along with the door guard who John injured but thought his arm was broken. There Molly tests Sherlock's urine for drugs and repeatedly slaps him for his reckless behaviour. They all realise that the drug-den door guard, Bill Wiggins, has impressive skills in the deduction.
Back at the apartment, Anderson helps John search for drugs, much to Sherlock's annoyance. Mycroft realises that Sherlock is working against Charles Augustus Magnussen, a newspaper owner who blackmails people and warns him not to. Sherlock is adamant and plans to meet Magnussen; however, his plan does not work out the way he wants it to. Magnussen visits Baker Street to intimidate Sherlock and refuses to enter negotiations over Lady Smallwood's stolen letters, and is careful to make Sherlock aware that he knows of "Redbeard", but avoids elaborating. The business magnate deliberately urinates in the fireplace, mainly to show that he can.
John is also baffled to find Sherlock in a relationship with Mary's bridesmaid, Janine until he learns that Janine is Magnussen's assistant and the liaison a ruse. Using her help, Sherlock and John break into Magnussen's apartment, only to find that Janine and a guard are unconscious. Sherlock goes upstairs to investigate but is shocked to find Mary holding Magnussen at gunpoint. Mary shoots Sherlock, who harnesses all his mental powers to stay conscious, including remembering his childhood dog, the mysterious "Redbeard", who was put down. He is taken to the hospital. In his mind palace, he is fighting to stay conscious through the pain while Moriarty mocks him. In the real world, Sherlock's vitals are dropping. His heart stops beating in surgery, while the doctors are prepping for surgery, and the doctors fail to revive him. When his "mind palace-self" fights back and escapes Moriarty, his real-world self makes a comeback. He wakes up in surgery (from pre-op sedation) and mutters "Mary". The next morning (only hours later) John announces to Mary that Sherlock is stabilised. Mary visits the steadied, yet in critical condition Sherlock, and tells him to keep the shooting incident between them. Sherlock fades away, falling back asleep.
Janine realises that Sherlock used her and retaliates by turning down his pain medication and talking to tabloids. She visits Sherlock, the afternoon after the shooting, to taunt him, now that he has stabilised and conscious. John begins to suspect something about Mary. John brings Lestrade to the hospital to question Sherlock about the shooting.
Sherlock runs away from the hospital and arranges a confrontation with Mary, who confesses to having a past as a secret agent and assassin, for which Magnussen is blackmailing her. It is also confirmed she intentionally shot Sherlock in a non-lethal spot and called an ambulance, and she says she is desperate to keep John in the dark as she loves him. John, however, witnessed the conversation and is in turmoil.
Back at Baker Street, Sherlock points out to John that he is addicted to a dangerous lifestyle and was most probably attracted to Mary because of her hidden past. The three talk and Mary gives John a memory stick containing information on her, marked with her right initials. The flash drive is labelled "A. G. R. A". She also tells him the contents will destroy his love for her. Sherlock then proves that Mary had called London "999" before John when Paramedics appear after being told that there was a shooting. Sherlock refers to a shooting that happened prior. With the paramedics present, Sherlock faints from pain from hypovolemia and is treated on the scene. In the apartment, while the paramedics restart Sherlock's heart, at his request, John thinks over what Mary and Sherlock had explained to him. On Christmas, after sitting on the memory stick for a few months, John decides to destroy it without reading it and to love Mary regardless, much to her relief.
(Although he does admit he is still "basically pissed off" with her.)
The couple spends Christmas with the Holmes brothers (and Wiggins as well) at their parents'. Sherlock drugs everyone, with the help of Wiggins except John and takes him to Magnussen's country property, Appledore, which is supposed to contain his blackmailing archive. During their confrontation, Magnussen explains his purpose in putting pressure on Mary was to arrive at Mycroft via John and Sherlock. Magnussen also reveals he was the one who had John abducted and hidden in a bonfire (in "The Empty Hearse") to confirm that John is Sherlock's "pressure point". Sherlock offers to buy Magnussen's information on Mary with the state secrets contained in Mycroft's laptop. Still, he realises immediately Sherlock is setting up a trap: security services looking for the portable computer will raid his vaults, find their contents and arrest him as a blackmailer. However, Magnussen triumphantly reveals that this plan can't work because Appledore's vaults don't physically exist, but are instead his mind palace. By giving Magnussen Mycroft's laptop, Sherlock and John are now guilty of attempting to sell government secrets and can be imprisoned for high treason, whereas Magnussen remains legally untouchable.
To pass the time until Mycroft's government forces arrive, he demonstrates his control over Sherlock and John by taunting them while repeatedly flicking John's face. When Mycroft and the police arrive, a distraught Sherlock shoots Magnussen in the head knowing its the only way to free them and everyone else from his power.
Sometime later, Mycroft convinces the government to spare Sherlock a trial and, as an alternative punishment, press him into a highly dangerous MI6 mission to Eastern Europe. John and Mary fondly bid goodbye to Sherlock, thinking that it is their last encounter, and Sherlock boards the plane. However, within minutes, TV screens all over England are hacked to broadcast a loop of Jim Moriarty asking "Did you miss me?" Thus Sherlock's flight is turned around, and he is summoned back.
- "I still don't understand."
"You should put that on a shirt."
- ―John and Magnussen
- "Oh, do your research. I'm not a hero; I'm a high functioning sociopath. Merry Christmas!"
- ―Sherlock, shooting Magnussen and ending the man's reign of terror
- "How's exile, brother dear?"
"I've only been gone four minutes."
"Well, I certainly hope you've learned your lesson. As it stands, you're needed."
"For god's sake! Make up your mind. Who needs me this time?"
(Moriarty's broadcast plays over and over)
- ―Sherlock is saved from a fatal assignment thanks to Moriarty's return
- The title of this episode refers to the Sherlock Holmes story, His Last Bow.
- The character of Charles Augustus Magnussen is based upon Charles Augustus Milverton from the Sir Arthur Conan Doyle story, The Adventure of Charles Augustus Milverton.
- When Janine mentions she purchased a cottage on the Sussex Downs (and there are bees), this is an allusion towards His Last Bow and The Adventure of the Lion's Mane, in which Sherlock has since retired to a small cottage, and has taken up bee-keeping.
- The beginning of the episode is taken almost verbatim from The Man with the Twisted Lip.
- Mary's true initials (A.G.R.A) is a reference to the treasure of Agra in The Sign Of Four, in which Mary is first introduced.
- The name of Sherlock's "protegé", Billy Wiggins, is a combination of the names of two helper characters from the books: Billy from "The Adventure of the Mazarin Stone" and Wiggins, one of the Baker Street Irregulars who helped Holmes during multiple cases including A Study in Scarlet.
- Sherlock calls Magnussen "the Napoleon of blackmail", an allusion to Doyle's Sherlock calling Moriarty "the Napoleon of Crime" in "The Final Problem".
- Mary Watson having had a previous life with a previous identity deliberately kept secret from her husband echoes the life of Mrs Elsie Cubitt in "The Adventure of the Dancing Men".
- In the closing dialogue of the episode, a story Mycroft told Sherlock about "the East wind" during their childhood, is similar to an ending speech from "His Last Bow".
- according to the episode commentary, the name "William Sherlock Scott Holmes" comes from the W. H. Baring-Gould’s "biography of Sherlock Holmes".
- In the credits of the episode on Masterpiece Mystery on PBS, some of the letters were highlighted red. Letters from this episode spell out M-I-L-V-E-R-T-O-N, a reference to the "The Adventure of Charles Augustus Milverton" which is a significant source of inspiration for this episode.
- In a scene cut from the episode, and added on the Special Edition of the Season 3 DVD, Magnussen comes to meet Sherlock in hospital, who is lying barely conscious, exhausted and heavily drugged in bed. Magnussen takes Sherlock's hand in his, caressing it and even kissing it, comparing it to an artist's hand "or a woman's." He apologises for his wet touch, but chillingly says, "you'll get used to it." He then leans closer to Sherlock, to the point where he is almost kissing him, and states his reasoning for not turning Mary over to the police, and then tells Sherlock he should also refrain before Sherlock loses consciousness and Magnussen finally leaves. Although difficult to watch, the scene is vital in showing the predatory nature of Magnussen, as well as a vulnerable side of Sherlock that is rarely shown.
- When John retrieves the tire iron from his car before entering the smack-house, there is a billboard behind him. The middle portion of the billboard has been torn down, revealing the old advertisement underneath. The combination of the new and old signs now read, "Information is power." This is the main plot of this story, as one man uses the information to maintain power over the world via blackmail.
- Sherlock says he won the properties in Leinster Gardens from the Clarence House cannibal: "Quite a gambler, that woman." Until her death, Clarence House was the official residence of the Queen Mother, who was a very keen gambler.
- When Sherlock asked why Mycroft needs him in England, he replied,“Here be dragons.”. Here be dragons means dangerous or unexplored territories, in imitation of the medieval, practice of putting dragons, sea serpents and other mythological creatures in uncharted areas of maps.
- The football match seen in the pub is a friendly match between Sheffield FC and Inter Milan Under-19 played to celebrate SFC's 150th anniversary in November 2007. However, the commentary is not from that match.
- When Sherlock was falling backwards, in the background of the shot, a plant moves across the room as if the room is tilting. Production designer Arwel Jones has said at least twice on his Twitter account that the plant was moved to give the effect that the room was spinning.
- There is a portrait of production designer Arwel Jones in CAM's mind place.
- Two graffiti of production designer Arwel Jones's initials are seen in the drug-den.
- Young Sherlock is played by Louis Moffat, son of the writer Steven Moffat and producer Sue Vertue.
- "His Last Vow" was the most tweeted about a single episode of a drama series ever.
- It's possible that Eurus Holmes played the Moriarty message at this time to save her brother from being sent away. Though insane, she does love him.
- After Magnussen's visit, Watson puts knocker tilted to the left, but in the last frame in that scene just when Sherlock's taxi leaves, the knocker is straight again. This is also seen when Watson is waiting with Sherlock for the taxi to arrive.
- In the scene in the lab where they're testing Sherlock for drugs, Sherlock's arm is folded in some scenes and down in some.
- Molly, when testing Sherlock for drugs, doesn't take her gloves off the proper way when dealing with hazardous substances.
- When Sherlock states that the guard is a former convict and white supremacist, he inverses the possible meaning of the tattoos (ie. 14 should mean white supremacist, and the five dots should mean prisoner).
- The helicopter that Sherlock and John used to go to Magnussen's house and Mycroft uses when the Police raids the house has the same registry number (G-VGMG).
- When Sherlock goes to access the elevator, he touches his access card to the reader like an embedded RFID tag instead of swiping or inserting the card through a slot, which is how a magnetic stripe reader would operate. If the elevator uses an RFID proximity reader, phone won't corrupt an embedded RFID tag.
- When Sherlock falls to his back, some support beams appear visible behind his shoe heels.
- When the surgeons revive Sherlock, the string holding his breathing tube changes positions multiple times.
- When Mary comes to the hospital and runs into John, he tells her that Sherlock had woken up and said: "Mary", however, Sherlock mutters "Mary" after waking up in the operating room, where John is not present.
- Benedict Cumberbatch – Sherlock Holmes
- Martin Freeman – John Watson
- Mark Gatiss – Mycroft Holmes
- Rupert Graves – Greg Lestrade
- Una Stubbs – Mrs Hudson
- Amanda Abbington – Mary Morstan
- Louise Brealey – Molly Hooper
- Andrew Scott – Jim Moriarty
- Jonathan Aris – Anderson
- Lindsay Duncan – Lady Smallwood
- Yasmine Akram – Janine
- Tom Brooke – Bill Wiggins
- Wanda Ventham – Mrs Holmes
- Timothy Carlton – Mr Holmes
- Calvin Demba – Isaac Whitney
- Tim Wallers – John Garvie
- Glen Davies – Chauffeur
- Brigid Zengeni – Kate Whitney
- Matthew Wilson – Security Man
- Louis Moffat – Little Sherlock
- David Newman – Medic
- Simon Kunz – Sir Edwin
- Katherine Jakeways – Benji
- Will Ashcroft – Club Waiter
- Ged Forrest – Security Guard
- Jamie Jarvis – Surgeon
- Lars Mikkelsen – Charles Magnussen
- Sherlock closes case in season finale with almost 8.8 million viewers, John Plunkett, The Guardian, 13, January 2014.
- TV Programme Information: Sunday 12 January, BBC, retrieved 28th December, 2013.
- Arwel Wyn Jones, Twitter
- Arwel Wyn Jones, Twitter
- Moriarty's return and Sherlock's girlfriend help His Last Vow break the Twitter record, Kay Dekker, Radio Times, 13 January 2014
|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|