Sherlock Holmes saves noted Swiss physicist Franz Tobel from the Gestapo in Zurich and smuggles him to London. There, Tobel is reunited with his wife, Charlotte, 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, Holmes and Dr. Watson risk their lives to find him.