Terror by Night is a 1946 Sherlock Holmes mystery film inspired by Sir Arthur Conan Doyle, loosely based on The Disappearance of Lady Frances Carfax and The Adventure of the Blue Carbuncle. It was directed by Roy William Neill.
Holmes and Watson meet Inspector Lestrade on a train to Scotland, in which Lady Margaret Carstairs, the owner of a famous diamond, the Star of Rhodesia, and her son Roland are travelling. The owner has asked Holmes to guard the huge diamond, but soon Roland is murdered and the diamond stolen. Holmes must find the diamond and uncover the murderer, both of whom must still be on the train.