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. Leaving Charlotte's apartment, an attempt to abduct him by German spies is foiled by a passing London bobby.
Tobel successfully demonstrates the bombsight for Sir Reginald Bailey (Holmes Herbert) and observers from Bomber Command. Tobel, now under the protection of Inspector Lestrade (Dennis Hoey) and Scotland Yard, tells Sir Reginald that, although willing to provide the British with his bombsight, only he will know its secret and has a complex plan for its manufacture to keep the secret safe. He separates his invention into four parts and gives one to each of four Swiss scientists, known only to him, to construct separately and whose names are unknown to each other. Soon after, Holmes receives a call from Lestrade telling him that Tobel has disappeared. Holmes goes to Charlotte's flat, where he receives Tobel's envelope. Rather than the coded message, the message inside is from Holmes' nemesis, master criminal Professor Moriarty (Lionel Atwill), who is now working for the Germans....
Directed by Roy William Neill
Written by
W. Scott Darling
Edward T. Lowe Jr.
Screenplay by Edmund L. Hartmann
Based on The Adventure of the Dancing Men
1903 short story
by Sir Arthur Conan Doyle
Produced by Howard Benedict
Starring
Basil Rathbone
Nigel Bruce
Lionel Atwill
Cinematography Lester White
Edited by Otto Ludwig
Music by Frank Skinner
Basil Rathbone as Sherlock Holmes
Nigel Bruce as Doctor Watson
Lionel Atwill as Professor Moriarty
Kaaren Verne as Charlotte Eberli
William Post Jr. as Dr Franz Tobel
Dennis Hoey as Inspector Lestrade
Holmes Herbert as Sir Reginald Bailey
Mary Gordon as Mrs. Hudson
Henry Victor as Dr. Frederick Hoffn