Sherlock Holmes (Basil Rathbone) and Dr. Watson (Nigel Bruce) square off against their archenemy, Professor Moriarty (George Zucco), who has designs on the Tower of London's Crown Jewels. As a perfect distraction from Moriarty's schemes, Holmes and Watson are contacted by a woman (Ida Lupino) who claims that her family is in danger after her brother receives the same warning message her father got 10 years ago -- just before he was brutally murdered.