In April 1946, Alicia Huberman, the American daughter of a convicted Nazi spy, is recruited by government agent T. R. Devlin to infiltrate an organization of Nazis who have escaped to Brazil after World War II. When Alicia refuses to help the authorities, Devlin plays recordings of her fighting with her father and insisting that she loves America.
Devlin and Alicia meet at the track, with Alexander watching from the grandstand.While awaiting the details of her assignment in Rio de Janeiro, Alicia and Devlin fall in love, though his feelings are complicated by his knowledge of her promiscuous past. When Devlin gets instructions to persuade her to seduce Alex Sebastian, one of her father's friends and a leading member of the Farben executives, Devlin fails to convince his superiors that Alicia is not fit for the job. Devlin is also informed that Sebastian once was in love with Alicia. Devlin puts up a stoic front when he informs Alicia about the mission. Alicia infers, mistakenly, that he was merely pretending to love her as part of his job.
Devlin contrives to have Alicia meet Sebastian at a riding club. He recognizes her and invites her to dinner where he says that he always knew they would be reunited. Sebastian quickly invites Alicia to dinner the following night at his home, where he will host a few business acquaintances. Devlin and Captain Paul Prescott of the US Secret Service tell Alicia to memorize the names and nationalities of everyone there. At dinner, Alicia notices that a guest becomes agitated at the sight of a certain wine bottle, and is ushered quickly from the room. When the gentlemen are alone at the end of the dinner, this guest apologizes and tries to go home, but another insists on driving him, implying that he will kill him.
Soon Alicia reports to Devlin, "You can add Sebastian's name to my list of playmates." When Sebastian proposes, Alicia informs Devlin; he coldly tells her to do whatever she wants. Deeply disappointed, she marries Sebastian.
Alicia takes the wine cellar key while Alexander dresses for the party. The gown is by Edith Head.After she returns from her honeymoon, Alicia is able to tell Devlin that the key ring her husband gave her lacks the key to the wine cellar. Devlin suggests that Alicia give a grand party and invite him, so he can investigate. Alicia secretly steals the key from Sebastian's ring, and Devlin and Alicia search the cellar. Devlin accidentally breaks a bottle; inside is black sand, later proven to be uranium ore. Devlin takes a sample, cleans up, and locks the door as Sebastian comes down for more champagne. Sebastian is shocked and dismayed to see the two of them alone together. Devlin pretends to be drunk and tells Sebastian that in his drunken state he insisted that Alicia come down to the cellar with him. She confirms this story and adds that she acquiesced in order to prevent Devlin from making an embarrassing scene in front of the party guests. Devlin congratulates Sebastian on having won Alicia's love and respect, and makes an exit.
Sebastian realizes that the cellar key is missing from his key ring, but the next morning he sees that Alicia has reattached it during the night. Thoroughly alarmed, he returns to the cellar and finds the glass and sand from the broken bottle.
Now Sebastian has a problem: he must silence Alicia, but cannot expose her without revealing his own blunder to the rest of the Nazi emigres, who will certainly kill him if they learn that he has married an American agent. When Sebastian discusses the situation with his mother, she suggests that Alicia "die slowly" by poisoning. They poison her coffee and she soon falls ill. Her illness worsens until she collapses and is taken to her room, where the telephone has been removed. Too weak to leave, she perceives that her husband is slowly poisoning her to death.
Devlin, alarmed by Alicia's continuing absence, sneaks into Alicia's room, where she tells him that Sebastian and his mother are poisoning her. Shocked into honesty by Alicia's peril, he finally confesses his love for her, and carries her out of the mansion in full view of Sebastian's co-conspirators. Sebastian and his mother go along with Devlin's story that Alicia must go to the hospital, but the conspirators are not convinced. Outside, Sebastian begs Devlin and Alicia to take him with them, but they contemptuously drive away, leaving him behind to suffer a grim fate at the hands of his fellow Nazis.
