An American operative in Great Britain (George Brent) and his counterpart from Scotland Yard (Basil Rathbone) suspect the beautiful singer Carla Nillson (Ilona Massey) of espionage. As they cleverly unravel her technique of singing in code over the radio, they track her from London, to Lisbon, to New York, where they succeed in tying her to a wealthy candy manufacturer who is, in reality, the sabotage mastermind.