Sherlock is lauded throughout Britain for his numerous triumphs of detection, the most famous of which was the recovery of Turner's masterpiece, "Falls of the Reichenbach." The media love him, his deerstalker hat, and his friend, "confirmed bachelor" John Watson. But wary of a tabloid culture that devours its own, John warns Sherlock to keep a low profile. Meanwhile, Sherlock's arch-nemesis, the criminal mastermind Moriarty, emerges from the shadows and into the spotlight with a menacing vow and a cat-and-mouse game conceived entirely for the object of his obsession. While John worries about a pending tabloid exposé of his friend and Moriarty's promised final problem, Sherlock continues to assist Lestrade solving crimes. But eventually even Sherlock must turn his absolute attention to the unhinged, evil genius' threat, "I owe you a fall."
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Warning: Contains significant plot spoilers
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Warning: Contains significant plot spoilers
John sits in his therapist's office, managing a grief-struck utterance, "My best friend, Sherlock Holmes, is dead."
Three months earlier, Sherlock is lauded throughout Britain for his triumphs of detection, the most famous of which is the recovery of Turner's masterpiece, "Falls of the Reichenbach." Meanwhile, Moriarty reappears, orchestrating a trifecta of master crimes: breaking into the Crown Jewels display at Tower of London, the vault at the Bank of England, and the security system at Pentonville Prison. Oddly, however, he takes nothing and allows himself to be caught.
Sherlock is asked to testify at Moriarty's trial, where he intellectually eviscerates tabloid journalist Kitty Riley, who was trying to entrap him. Moriarty, after threatening sequestered jurors, is acquitted, and walks free — directly to 221B Baker Street for a tete-a-tete with Sherlock. Sherlock realizes that Moriarty stole nothing because he possesses something far better: a key code allowing him to break into any security system in the world. But when pressed, before departing, Moriarty reveals his true motive for his stunt: resolving, with Sherlock, what he calls "the final problem."
Mycroft arranges a meeting with John to warn him of Kitty Reilly's pending Sun exposé about Sherlock, and of four assassins who have recently relocated around 221B Baker Street. Mycroft suspects Moriarty and asks John to look out for Sherlock.
Meanwhile, Sherlock is assisting Lestrade in finding the British Ambassador's children, kidnapped from their boarding school. From footprints found at the school with traces of glycerine; an edition of Grimm's fairy tales; and two breadcrumb-filled envelopes, one at 221B and one at the school, Sherlock concludes that Moriarty has created his own fairy tale. The glycerine, used in producing chocolate, will lead him to the kidnapped children. But when they find the children, who have been fed mercury-poisoned candy, the girl utters a horrific scream at the sight of Sherlock. Puzzled, Sherlock is riding home in a cab when a video starring Moriarty as a children's TV character commences, telling the story of Sir Boast-a-Lot, whose amazing feats inspire doubt...and then suspicion. Sherlock bolts from the cab only to discover the driver: Moriarty. In pursuit, he is rescued from a near-collision with a car, but his rescuer is immediately killed by a sniper. John identifies him as one of the assassins and Sherlock realizes that the assassins are there to keep him alive because he has something that all of them want.
Simultaneously, Donovan informs Lestrade of her suspicion that Sherlock engineered the kidnapping himself in order to gain publicity. Now, Sherlock understands Moriarty's plan: to destroy him by exposing him as a fraud. The Chief Superintendent orders Lestrade to bring him in, but Sherlock and John escape. As they flee, an assassin, in their pursuit, reveals that Moriarty left the computer key code in Sherlock's apartment. Before returning to 221B to find it, they stop at Kitty Reilly's home to learn about the source of her exposé. As they question Kitty, Moriarty arrives. She introduces him as Rich Brook, the actor Sherlock hired to play Moriarty — an invented villain to take the blame for invented crimes. Terrified of Sherlock, "Rich" flees and disappears. Sherlock now understands the final problem. Alone, he finds Molly at the lab and asks for her help.
Orchestrating his confrontation with Moriarty, Sherlock tricks John into leaving him alone. On the rooftop of St. Bart's Hospital, the enemies meet and Sherlock recounts his full understanding of Moriarty's plot: "Richard Brook" is the German of Reichenbach, the case that made Sherlock's name. And the key code is a binary code which Moriarty tapped out with his fingertips at 221B. But Moriarty discloses that there was never a code — the robberies were pedestrian crimes. Crushed at having so easily tricked the only person he thought a worthy challenge to his evil genius, Moriarty reveals that the fall he promised is Sherlock's own: Sherlock must kill himself, or the assassins will kill John, Mrs. Hudson and Lestrade. But Sherlock realizes that Moriarty must possess a code to call off the killers once he has died, and with Moriarty alive, he can prevent his friends' deaths. His revelation that he is prepared to go to any length imaginable to save his friends satisfies Moriarty's obsession that they are the same after all. He puts a gun in his mouth and blows his brain out. Now Sherlock is left powerless but to take his own life to save his friends'.
As John makes his way to join the pedestrians gathered on the street below, Sherlock calls him, confesses that he fabricated everything, and has John stop to watch his jump. He leaps off the roof and falls to the pavement, and as John runs to him, a passing cyclist knocks him down. Dizzily hurrying to Sherlock's body, he finds no pulse. The paramedics rush Sherlock away.
But as John cries at Sherlock's grave, saying his last goodbye and wishing for a miracle, Sherlock stands hidden at a distance. He watches his friend depart.