Blackmailed and sidelined from investigating, Sherlock Holmes finds hope when a young woman arrives on his doorstep, potentially holding the key to solving the kidnapping of an ambassador's daughter.

Amelia goes under cover to search for clues at Clara's home; Sherlock goes to prison under pretense of meeting his forever foe, Moriarty.

Amelia withholds details from Holmes after encountering a charming man with a pistol, but Sherlock must step in when her investigation turns fatal.

Sherlock searches for the missing jewels and their connection to the kidnapping of his friends; he turns to the insurance investigator, Birtwistle, for his assistance.

As Sherlock and Amelia identify the villains surrounding them on all sides, Holmes repays a favor to his oldest enemy, arranging a meeting between Professor Moriarty and the young Australian man who has suddenly become very close to Amelia.