In London, Vivian Vedder (Renee Godfrey) verifies that a carpenter has completed a coffin for her recently deceased mother's body, which she is transporting to Scotland by train. She boards the train that evening, as do Lady Margaret Carstairs (Mary Forbes), who owns and is transporting the famous ''Star of Rhodesia'' diamond; Lady Margaret's son Roland (Geoffrey Steele); Holmes, whom Roland has hired to protect the diamond; Inspector Lestrade (Dennis Hoey), who is also worried about the diamond's safety; and Watson and his friend Major Duncan-Bleek (Alan Mowbray). Holmes briefly examines the diamond.