|Basil Rathbone||Sherlock Holmes|
|Nigel Bruce||Dr. Watson|
|Ida Lupino||Ann Brandon|
|Alan Marshal||Jerrold Hunter|
|George Zucco||Prof. Moriarty|
|E.E. Clive||Inspector Bristol|
|Mary Gordon||Mrs. Hudson|
Sherlock Holmes and Dr. Watson square off against their archenemy, Professor Moriarty, 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 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.