Alicia Huberman is attempting to get over the conviction for treason of her father, who was a member of the Nazi party. On the day he is convicted, she throws a party with her friends and gets intoxicated. The only unusual thing about the party is the stranger named T.R. Devlin who seems to have invited himself.
After the party, Alicia—visibly intoxicated—drives Devlin around, swerving and speeding, until finally they are pulled over by a traffic cop. Devlin gives the officer a flash of some credentials and suddenly they are free to go. When Alicia realizes that Devlin is a cop, she becomes livid, hitting him and accusing him of spying on her. Rendering her unconscious, Devlin drives Alicia home and puts her to bed.
When Alicia wakes up, hungover from her bender, Devlin tells her that they want her to infiltrate a cohort of Nazis settled in Rio. They all knew her father, which makes her an excellent candidate for the espionage. At first Alicia refuses, assuring Devlin that she is no patriot, but then Devlin plays her tapes they secretly recorded of her speaking with her father, in which she explicitly denounces his Nazi sympathies and declares her love for the United States of America. Eventually, she agrees to go to Rio with him.
In Brazil, Devlin falls in love with Alicia and they begin a passionate affair. Soon, however, Devlin gets word from his superior, Prescott, that Alicia is required to insinuate herself romantically with a former associate of her father's, Alexander Sebastian. Devlin does not prevent Alicia from accepting the task, which she misreads as his not caring enough about her to ask her not to do the job. When she agrees to go through with it, Devlin in turns reads her decision as confirming his doubts about her love for him.
Alicia insinuates herself with Alexander Sebastian, who invites her over to dinner at his house. While at dinner, Alicia notices one of the guests, Emil Hupka, points at a wine bottle with unusual alarm. After dinner, Emil’s improper display is the topic of conversation among the men, and one of the Nazis, Mathis, decides that he ought to kill Hupka.
At the racetrack, Alicia tells Devlin that she has seduced Sebastian, which seems to upset him. When Sebastian approaches them suspiciously, Alicia assures him that Devlin is in love with her, but that it is not reciprocated. Sebastian believes her. Soon after, Alicia informs Devlin and his boss that Sebastian has proposed. Devlin is very much taken back by this turn of events, but again says nothing. Utterly heartbroken, Alicia goes through with the wedding. Following a quick honeymoon, they return to set up home with Sebastian's overbearing mother, a dour German woman who inherently distrusts Alicia.
Alicia takes advantage of her opportunity to fully explore the house and eventually discovers that there is only one key that opens the door to the wine cellar, a key that Alexander never lets far from his sight. Devlin instructs her to encourage Sebastian to throw a party, steal the key, and explore the wine cellar with him while the party is going on. Before the guests arrive, Alicia is given a brief window of opportunity to retrieve the key off her husband’s key ring without him noticing. During the party, she and Devlin sneak down to the cellar. The secret of the cellar, they eventually discover, is that not all the bottles are filled with wine. Several of the bottles are filled with a sand that will ultimately be identified as uranium ore. Devlin accidentally drops one spilling the contents on the floor. They swiftly clean up the mess, but as they attempt to escape, Sebastian and a butler see them in the hallway. Hoping to deflect the attention away from the cellar, Devlin kisses Alicia in plain view of Sebastian.
Alicia assures Sebastian that Devlin took advantage of her because of his unrequited love, but that she does not return his sentiments. He believes her for a time, but later notices that the key to the wine cellar is missing. When the key mysteriously appears again on his ring the next morning, he makes a quick inspection of the cellar and discovers evidence of tampering. From this he concludes that Alicia is an American spy.
When Sebastian tells his mother about his discovery, she is livid and anxious that the other men in his cohort will find out how foolish he has been and kill him immediately. She assures him that any change in attitude toward Alicia would reveal Sebastian's misstep to Mathis. The only obvious course of action is to maintain appearances while poisoning Alicia to death over a period of time.
Over the course of the next few weeks, Sebastian and his mother poison Alicia's coffee. When Alicia goes to visit Devlin's boss Prescott, he gives her two pieces of news: the bottles are filled with uranium and Devlin has requested to be transferred off the case. Alicia goes to meet Devlin and confronts him about the fact that he is leaving, but he denies that he is leaving and is very concerned about Alicia's health. The poison has incapacitated her and makes it seem as though she is drunk. Over coffee at Sebastian's mansion, Alicia realizes that Sebastian and his mother are poisoning her, but is unable to escape. They lock her away in her bedroom without a telephone.
When Alicia doesn't show up for their next scheduled contact meeting, Devlin becomes suspicious and insists to Prescott that he ought to go drop by the Sebastian residence. When he arrives at the mansion, Sebastian is in a meeting with Mathis and the other Nazis, so Devlin takes the liberty of wandering up to Alicia's room, where he finds her drugged and ill. They each confess their profound love for the other, before Devlin begins to carry Alicia out of the house. They run into Sebastian and his mother in the hall. As Mathis and Sebastian's other Nazi associates come out of the room and ask what's going on, looking more and more suspicious, Devlin tells them that he is taking her to the hospital, but Sebastian is nearly speechless. Sebastian plays along and walks out to the car with Alicia and Devlin, but when he tries to get in, Devlin refuses to let him get in, revealing to Mathis that Alicia was an American spy all along. Devlin and Alicia drive away, and Sebastian goes into the house to face the man who it is now clear will be his executioner, Mathis.