Because her father was a German spy who has committed suicide in prison, Government agent T.R. Devlin recruits Alicia Huberman to become an undercover agent. She has fallen in love with Devlin and is trying to repay her father's moral debt to America, the country she loves and feels her father has betrayed. Her assignment in Rio de Janeiro involves resuming an acquaintance with a wealthy German businessman, Alexander Sebastian, who has been attracted to her. She is to infiltrate his circle of German scientists. Against the wishes of his mother, Sebastian asks Alicia to marry him. She accepts, and they marry, though she is disappointed when Devlin raises no objections. After their marriage, Alicia explores the Sebastian mansion, but finds the wine cellar is locked. Devlin tells Alicia to invite him to a party at Sebastian's mansion. She is to take the key to the wine cellar from Sebastian's key ring without his knowing it. At the party, Devlin and Alicia investigate the wine cellar, where the find some bottles of sand. When Sebasian finds them together in the wine cellar, they convince him they are having a tryst. Noticing that the key was missing and then returned, Sebastian suspects Alicia of being a U.S. spy and his mother plots to eliminate her because she has become a woman who knows too much.