This episode follows after The Abominable Bride.
After being put on release, Sherlock is called in to investigate a week-old corpse found rotting in a car. The car was hit hours earlier by another car, and the police on the scene find the body.
This leads to a series of Margaret Thatcher bust being smashed, the motive unknown. But little does Sherlock realize that what seems to be a petty crime turns into a larger problem that has ties to the group of assassins, A. G. R. A.
The episode opens with a monologue from Sherlock telling the story of a merchant who comes into contact with Death as a figure and despite his attempts cannot escape fate. Mycroft briefs Sherlock on the new 'official' story of Charles Augustus Magnussen, who was killed by Sherlock. Despite the seriousness of the situation, Sherlock goofs around and ends up asking the Secretary, Vivian Norbury, about ice-lollies. John and Mary Watson welcome the arrival of a baby daughter, whom they name Rosamund. At the same time, following the events of "The Abominable Bride", Sherlock has become obsessed with working out his arch-enemy Moriarty's posthumous revenge, with Mycroft and various Scotland Yard detectives including Greg Lestrade assisting him in solving seemingly trivial cases, hoping it will lead them to a larger scheme.
He investigates a case where the son of a rich couple (Charlie Welsborough) is found dead in a car wreck, despite seemingly being on a gap year in Nepal. Sherlock quickly solves the case, but is then led into another mystery involving the smashing of a bust of Margaret Thatcher. Further busts are smashed and upon encountering and fighting the culprit, Sherlock discovers that the last bust contained a memory stick containing information about Mary and her past as a government agent. A figure from her past is bent on revenge in the belief that Mary betrayed him.
Upon questioning from Sherlock, Mary explains that she and her colleagues were members of a freelance task force named 'A.G.R.A' (derived from the members' names, Mary being 'R' for Rosamund - her real name - and A, her pursuer, Ajay). All members have a memory stick containing data on each other to force an inability for one to betray the other. (If one released information on the other, the victim could do the same likewise). A failed rescue mission of members of the British embassy in Georgia under coup d'etat (set off by codeword 'Ammo') resulted in the embassy members dead and Mary fleeing.
After informing Sherlock, Mary knocks him out with a drugged letter and runs away across the world, following a random path. Sherlock and John quickly find her, having placed a tracking device on the memory stick. Ajay has also followed their lead. He explains that he was captured by the terrorists but not before hiding his memory stick in one of the Thatcher busts which he intended to recover. During his torture Ajay heard whispers of 'Ammo' and 'the English woman' being the reason for the mission failure. Ajay attempts to kill the trio but is shot by police. They reflect on what has happened and believe that Ajay was working on the assumption that 'the English woman' was Mary.
Sherlock visits Mycroft and discusses the situation. Mycroft explains that he previously hired 'A.G.R.A' but hasn't since the failed mission. Mycroft also states he has no knowledge of 'Ammo'. Later Sherlock calls Mycroft and explains that 'Ammo' is really Amo, the Latin word for "I love". Suspect Lady Smallwood, codename Love, is then interrogated by Mycroft. Sherlock later pieces together the final part of the puzzle when he recalls Mary explaining the great thing about 'underlings' bringing information but still being under everyone's noses. He meets Vivian Norbury in the London Aquarium. Vivian reveals that she tipped off the terrorists on the rescue so that the hostages and 'A.G.R.A' could be eliminated.
Her motive was to kill the British ambassador who found out Vivian was selling secrets and to achieve a 'quiet life'. When the police and Mary arrive, Vivian pulls out a gun and shoots at Sherlock. Mary jumps in the way, taking the bullet. John appears and Mary tells of her love for being 'Mary Watson'. She then dies in John's arms. Since Sherlock had vowed to protect her, this puts a rift between him and John Watson. Sherlock visits a therapist's but is reluctant to converse. Returning to Baker Street, Sherlock and Mrs. Hudson lament the loss. Sherlock opens a parcel containing a DVD; on it is a posthumous message from Mary. She instructs him to 'Save John Watson'. Returning to the story of the merchant, Sherlock concludes that death is inevitable but questions if how one dies is also certain and unchangeable. In a post-credits sequence, additional footage from Mary's DVD shows her scathing 'Go to Hell Sherlock'.
- The title of the episode is a reference to the story "The Adventure of the Six Napoleons".
- At the end of the episode Sherlock tells Mrs. Hudson : "If it should ever strike you that I am getting a little overconfident in my powers, or giving less pains to a case than it deserves, kindly whisper 'Norbury' in my ear, and I shall be infinitely obliged to you". It's a reference to the story "The Adventure of the Yellow Face".
- When Sherlock and John track down Mary, John says "Mary, I may not be a very good man, but I think I'm a bit better than you give me credit for." This is another reference to "The Adventure of the Yellow Face".
- The story Sherlock narrated in the beginning and the end is a version of a Mesopotamian tale which is retold by W.S. Maugham called the "Appointment in Samarra" at the end of a play called "Sheppey" which is written by Maugham; with a difference where Maugham writes that the Merchant's servant encountered Death, not the Merchant himself. A version of said story can be found here. Sherlock personally have never liked the story, with Mycroft even noting that Sherlock wrote a version of the tale where the Merchant became a Pirate.
- When Sherlock looks at a case called "The Duplicate Man", he replies to Dr. Watson's unspoken suggestion with "It's never twins" (a catchphrase repeated from The Abominable Bride). The scene followed by a new case "The Circus Torso", where he orders people around "Hopkins, arrest Wilson" and "Dimmock, look in the lymph nodes". This is likely a nod to the TV-series "House M.D." where Dr. Gregory House is an alternate universe Sherlock Holmes, and Dr. James Wilson is his Dr. Watson counterpart, solving medical mysteries like Sherlock solves crimes. Dr. House's catchphrase is "It's never Lupus".
- A glimpse in one of he cases Holmes blazes through is "Fresh paint to disguise another smell." A clear reference to "The Adventure of the Retired Colourman". There the heavy smell of paint served to camouflage the recent use of gas to commit murder.
- 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
- Siân Brooke – Eurus Holmes
- A.G.R.A. is not Mary's initials entirely; she is only R.
- Ajay blamed Mary because she was the only "English woman" he could think of.
- When Sherlock is hit by the knockout drug by Mary, he briefly has vague visions of Eurus and her song.
- Magnessen's death is presented to the public as due to a sniper with an itchy trigger finger.
- Sherlock raising the gun and shooting him is cut from the footage.
- Mary jumps in front of Sherlock, instead of pulling him away from the gunshot.
- Given how fast she moved, she could have easily moved him to minimize the damage from the bullet.