Disguised as an old bookseller, private detective Sherlock Holmes pretends to be a Nazi spy in order to help Swiss scientist Dr. Franz Tobel escape from a Gestapo trap in Zurich. Holmes and Franz take a British transport plane to London, where the physicist leaves the safety of Holmes's Baker Street address for a secret meeting with his wife, Charlotte Eberli Tobel. Franz gives her an envelope containing a coded message, and asks Charlotte to give it to Holmes should anything happen to him. Having been unknowingly tailed to Charlotte's apartment by German spies, Franz is saved from abduction when a London bobby happens by. The next day, Franz successfully demonstrates his new bombsight for Sir Reginald Bailey and the British air ministry. Back at Whitehall, Franz tells Sir Reginald that, while he is willing to allow the British to use his invention, only he will know its secret. Franz then splits his invention into four parts and hires four Swiss scientists living in London to construct separately each part. Soon after, Holmes receives a call from Scotland Yard's Inspector Lestrade telling him that Franz has disappeared. Holmes and his physician assistant, Dr. John H. Watson, go to Charlotte's apartment, where the young lady gives the detective Franz's envelope. Rather than the physicist's coded message, the message inside is from Holmes' nemesis, master criminal Professor Moriarty. Holmes then goes into the Soho district, disguised as Ram Singh, one of Moriarty's old henchmen. Led to the criminal by Moriarty's own henchmen, Peg Leg and Jack Brady, Holmes pleads with Moriarty to release Franz for the good of England, but Moriarty refuses and gloats over his victory over his longtime foe. Holmes is forced into the false bottom of an old sea chest, but is rescued from a watery grave when Watson and Lestrade discover Brady and Peg Leg struggling with the chest. Holmes then rushes back to Charlotte's apartment, where he discovers the tracings of Franz's coded message on a notepad. With Watson's help, Holmes breaks the first three lines of the code, which are the names and addresses of three Swiss scientists, but soon learns that all three men have been murdered. Meanwhile, Moriarty, unable to break the fourth coded line, tortures Franz for the name of the fourth scientist. Back on Baker Street, Holmes breaks the fourth line's code and rushes to the home of Professor Frederick Hoffner. At the same time, Moriarty deciphers the code, but rather than sending his Nazi assistants to kill Hoffner, Moriarty decides to abduct the elderly Swiss scientist, as insurance should Franz fail to recover from the torture. The German agents, however, bring a disguised Holmes to Moriarty's hideout. Taking Holmes's own suggestion, Moriarty straps Holmes to an operating table and slowly starts to bleed the detective to death. Holmes is saved at the last minute, however, by Watson and Lestrade, who have followed a trail of fluorescent paint droppings to the criminal's waterfront hideout. The spies are captured, and when Moriarty attempts to make his escape through a secret passageway, he falls, seemingly, to his death, as Holmes had previously discovered the criminal's hidden trap door. With Franz and his bombsight safely in English hands, Holmes and Watson watch as a squadron of British planes head for Germany.