In London, Lady Margaret Carstairs (Mary Forbes) and her son, Roland (Geoffrey Steele), engage renowned detective Sherlock Holmes (Basil Rathbone) to accompany them back home to Scotland as protection for their priceless gem, the Star of Rhodesia. When Roland turns up murdered and the Star disappears, Holmes and his friend Dr. Watson (Nigel Bruce) investigate -- only to discover another murder, and a strange connection with Watson's acquaintance Maj. Duncan-Bleek (Alan Mowbray).