The Valley of Fear is the final Sherlock Holmes novel by Sir Arthur Conan Doyle. The story was first published in the Strand Magazine between September 1914 and May 1915. The first book edition was published in New York on February 27, 1915.

The plot of the novel is based very loosely on the real-life activities of the Molly Maguires and particularly of Pinkerton agent James McParland.

A Pinkerton agent infiltrates a criminal gang under the alias of John McMurdo and warns the gang that he has had a tip off that Pinkerton's detectives are onto them, even giving them his own true name, Birdy Edwards, for the agent that is on the case. He devises a plan with the gang to trap Birdy Edwards. He gives the gang the impression that he is dealing with an informant at Pinkerton's headquarters and obtaining up to date bulletins on Birdy Edwards's activities. When the gang are finally waiting for Birdy Edwards to arrive in the trap, McMurdo informs them that he is Birdy Edwards and they are all arrested with the help of the police who come in from outside the building. The actions of bringing the gang into the situation in which they were arrested provided compelling evidence of their guilt.