Sherlock Holmes and Dr. John Watson must unravel a complicated case of murder and mistaken identity after Holmes receives a warning from an agent in Professor Moriarty's employ that John Douglas, a man unknown to Holmes, is in danger. But when they are called to assist with the murder investigation of Mr. Douglas, Holmes and Watson must decipher the significance of events from twenty years in the past before they can uncover Moriarty's link to Mr. Douglas and his death.