Military Intelligence Service Director Mycroft orders Albert to recover classified papers that could rock the nation, and kill the woman who stole them. Sherlock is hired by the king of Bohemia to pursue Irene Adler, who threatens the king's wedding plans.

Following William's plan, Albert invites Irene to a masquerade, where a game with the famed "Lord of Crime" as its theme is being played. Dressed as a man, Irene uses the game to contact Albert, pleading for her safety in return for the stolen papers.

Despite Sherlock recognizing that Irene is dealing with the Lord of Crime, he sees her off. Irene negotiates with the Lord of Crime in an abandoned church, but before she can turn over the papers, Sherlock arrives, and makes a deal to keep Irene safe.

Receiving a new name, Bonde joins the Moriarty ranks, and is sent to retrieve the contents of a safe deposit box with Moran, Louis, and Fred. But when an unexpected bank heist spurs the others into spontaneous action, Bonde is left wondering what to do.

A series of murders is being carried out by someone claiming to be Jack the Ripper. In response, a vigilante group of the town's poor confronts Scotland Yard while William and the real Jack scramble to uncover the true culprit's aims and foil their plans.

A culprit is arrested in the Jack the Ripper case which was solved by the Moriartys. Suspecting a frame-up by Arterton, Lestrade asks Sherlock for help in finding evidence that would prove it. Meanwhile, Paterson works to sneak Bonde inside the station.

William outshined the adults in knowledge and nerve while saving the orphanage where he was staying. Judicial records state that William lent a nobleman £600 in the spirit of a certain play. But when the contract is broken, William takes drastic measures.

Adam Whiteley, a young MP in the House of Commons working toward an equitable society, garners public support as a "White Knight." Milverton is hired by those opposed to Whiteley to oust him. William takes interest in Whiteley, and decides to test him.

Whiteley goes alone to deal with the House of Lords with the irrefutable proof obtained from Albert in hand, but finds Milverton, the king of blackmailers. Even as Whiteley negotiates with him, Milverton's devilish clutches close in on his dear brother.

John introduces Sherlock to Mary, his fiancée. A stunned Sherlock realizes that Mary has another concern. She produces six pearls, detailing their connection to her missing father. She asks Sherlock to look into the matter, but is holding something back.

Milverton blackmails Mary, exploiting her dalliance in antigovernment activities as a student. Sherlock decides to sneak into Milverton's estate to steal his evidence, but even that act was anticipated. Sherlock finds that he is not the only trespasser.

When William James Moriarty claims responsibility for being the Lord of Crime in all the papers, aristocratic condemnation and public anger falls upon the Moriartys. The final problem of the Moriarty Plan that William has concocted at long last begins.

William aims to complete his plan with his death, as Albert and the others each carry out their roles. Meanwhile, Sherlock deduces William's true reason for having him vanquish the Lord of Crime. Overlooking London, the pair face each other one last time.