A mysterious Chinese woman arrives at Charlie Chan’s home and is murdered there shortly afterward. The only clues are the ring with which she introduced herself, and the message “Capt K” she scrawled before dying.
Joaquin Shannon confesses to a murder he didn’t commit. He gets a year in prison to start for manslaughter. It gets hard to be on good behavior.
The Society of the Black Dragons sends Bela Lugosi (Dr. Melcher) to transform 6 japanese into indentical likenesses of American industrialists forming a 5th column. This is at the behest of the Nazis.
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.