Sherlock Holmes and Doctor Watson are investigating a mysterious murder. John Douglas, a wealthy man with a mysterious and unknown past, is the victim. Suspicions of committing the murder fall on, among others, a friend of the house, the victim's spouse, a mysterious avenger... The details of Douglas's death are revealed in the second part of this intriguing novel. Its plot takes us from England to the United States, to the titular Valley of Fear, a place of dreadful crimes committed by members of a Masonic association inspired by the genius of evil, Professor Moriarty.