Sherlock Holmes (Arthur Wontner) and Dr. Watson (Ian Fleming) travel to Birlstone Castle to unravel a mysterious murder. The trail leads to a secret American society of coal-miners called the Scowlers. As usual, that diabolical villain Professor Moriarty (Lynn Harding in fine form) is the evil instigator who has allied with an American mobster to kill a Pinkerton detective, who is trying to bring down the Scowlers.