In search of Dr Keller, the man who stole the papers containing secret information on German air strength, Hop and Walters, the American agent, followed two Gestapo officers to Keller's home determined to get the papers back. Hop suspects that Keller is playing a lone hand, out to get as much as he can for the information - but he doesn't know that Keller has made arrangements with the Gestapo to double-cross the boys...