Sherlock Holmes (Basil Rathbone) saves noted Swiss physicist Franz Tobel (William Post Jr.) from the Gestapo in Zurich and smuggles him to London. There, Tobel is reunited with his wife, Charlotte (Kaaren Verne), to whom he gives a coded message in the event he disappears. Tobel presents the weapon he developed to government officials, and when he is subsequently abducted by arch-villain Professor Moriarty (Lionel Atwill), Holmes and Dr. Watson (Nigel Bruce) risk their lives to find him.