Sherlock Holmes (Basil Rathbone) pretends to be a Nazi spy to aid scientist Dr. Franz Tobel (William Post Jr.) and his new invention, a bombsight, in escaping a Gestapo trap in Switzerland. Holmes and Franz fly to London, where Holmes places him under the protection of his friend, Dr. Watson (Nigel Bruce). The scientist slips away against Holmes' instructions for a secret reunion with his fiancee, Charlotte Eberli (Kaaren Verne), and gives her an envelope containing a coded message. He tells Charlotte to give it to Holmes if anything should happen to him. German spies' attempt to abduct Tobel as he leaves Charlotte's apartment is foiled by a passing London bobby.