Holmes keeps his adversary Moriarty from selling a bomb-sight to the Nazis in this updated version of a story by Arthur Conan Doyle.
In Switzerland Sherlock Holmes (Basil Rathbone) in a disguise talks to two Nazis about a bomb-sight they want. Holmes calls on the inventor Dr. Franz Tobel (William Post Jr.) and uses his servants as decoys. Holmes and Tobel steal a car and are picked up in a plane.
In London Holmes hides the papers in his office. Tobel does not take the sleeping pill that Dr. Watson (Nigel Bruce) gives him and later makes a call to meet someone. Tobel gives Charlotte Eberli (Kaaren Verne) an envelope to give to Holmes if he dies. Tobel is attacked on the street but is helped by a policeman. Tobel comes back, and Holmes tells about the woman and asks about the attacker. From a plane Tobel tests his bomb-sight and is congratulated; but Tobel tells Sir Reginald (Holmes Herbert) he is keeping the secret of his device.
Tobel gives one of the four parts of his invention to the Swiss scientist Frederick Hoffner. Holmes says that Tobel is gone and learns Charlotte's address from the police by tracing Tobel's call. She won't say where Tobel is. When she learns that the car she loaned him was wrecked, she gives Holmes the envelope. Someone replaced Tobel's note, but Holmes figures out it was Professor Moriarty. Holmes disguises himself as a dockworker and visits bars. He buys information and finds Moriarty (Lionel Atwill), who has him tied up and put in a trunk. Watson and Inspector LeStrad (Dennis Hoey) rescue Holmes as the two men carrying the trunk flee.
Holmes and Watson go to Charlotte and find an impression of a code that Holmes deciphers, learning three of the four scientists. They find one dead, and Holmes says the other two must be dead too. LeStrad calls and confirms this. Moriarty tortures Tobel to learn the fourth man. Holmes deciphers the fourth name before Moriarty does. Moriarty sends two men to get Frederick Hoffner, but the man brought to Moriarty is Holmes in a disguise. Watson and LeStrade follow a trail left by Holmes. Moriarty plans to kill Holmes, who gets him to do it slowly. Watson and police see Tobel being carried out and sneak in to save Holmes and arrest Moriarty, who escapes; but Holmes arranges for him to fall through his trap door.
In this mystery the cleverness of Holmes has the patriotic
theme of saving England from improved German bombing.