U.S. State Department diplomat Barry Corvall (Joel McCrea) becomes a one-man counterspy agency in the dawning days of World War II. After Barry discovers that his new bride, refugee Brenda Ballard (Brenda Marshall), has been coerced into acting as an agent for Nazi Germany, he is expelled from the Foreign Service. Recruited by an Assistant Secretary of State, Barry goes undercover as a disgraced ex-diplomat, besotted by his Mata Hari wife. Journeying to Geneva, Barry and Brenda put themselves in harm's way in a desperate bid to uncover the secrets of the Nazi spy ring operating inside the United States. While Brenda continues to pose as a spy, Barry finds the proof he needs.