The film opens with a German air raid over the skies of London, and moves to the attempts of the F.B.I. and Scotland Yard investigators trying to circumvent the attempts of a sabotage ring dedicated to impeding the flow of American airplanes and flying fortresses to Britain (on FDR's Lend-Lease program since the United States was not yet at war with Germany and Italy.) Tim Hanley is an American agent, posing as a lawyer connected with the United States Embassy in London, and Reggie Oliver, a Scotland Yard detective, posing as a music critic, who has a hard time understanding American slang, are keeping their eye on Carla Nillson, a famous singer, whom they suspect of espionage. They all meet in London, then in Lisbon, and eventually in New York City, where Carla sings on the radio under the auspices and sponsorship of Sidney Grenner, a wealthy candy manufacturer, who is in reality the head of the sabotage gang. Miss Nillson may or may not know that some of the songs she sings over the radio are in code, and give instructions to the enemy operatives about airplane shipments to England. And even if she doesn't know, will she still be implicated and subject to being arrested by Tim, with whom she has fallen in love? And if she is arrested, will Tim be waiting for her after the Allies defeat the Nazis?