Affluent widow Lady Margaret (Mary Forbes) and her son Roland (Geoffrey Steele) enlist the help of Sherlock Holmes (Basil Rathbone) to keep a huge diamond from being stolen on a trip to Edinburgh. Holmes watches the precious jewel with help from Dr. Watson (Nigel Bruce) and Inspector Lestrade (Dennis Hoey), but all is turned upside down when the stone vanishes and Roland is killed. Now, Holmes must find the murderer among the train's passengers.