In London, at the end of World War II, Lieutenant Lawrence Gerard, a distinguished pilot and ex-prisoner of war, is discharged from the RCAF and applies for a passport to France to settle the estate of his French bride, a victim of the war.  Told that he must wait for the issuance of a passport, the determined Gerard rows a boat across the English Channel to France.

Once ashore, he goes to the office of his father-in-law, M. Rougon, the local prefect, and demands to know how his wife, Celeste, died.  When Rougon replies that Celeste was one of fifty patriots ordered executed by Marcel Jarnac, an official of the Vichy government, Gerard becomes overwrought and swears revenge on Jarnac.  Rougon explains that although Jarnac has been declared dead, he believes that the man is still alive because no one has ever seen his face, and therefore he cannot be identified.  Rougon sends Gerard to a police station in Marseilles, and while he is there, a call comes in that the police have Jarnac cornered in a house in a nearby village.  By the time Gerard and the others arrive, however, all that survives of the village is the charred remains of a house.  In the rubble, Gerard finds the title page of a dossier on Jarnac and an envelope addressed to Madame Madeleine Jarnac from an insurance company in Berne, Switzerland.

Posing as a friend of Mme. Jarnac, Gerard travels to Berne and presents himself at the office of the insurance company.  Although the company refuses to divulge Mme. Jarnac's whereabouts, they provide Gerard with an address that will forward her mail.  After stealing some stationery from the office, Gerard writes to Mme. Jarnac and then goes to the mailbox to retrieve the envelope with her forwarding address written on it.  The address is in Buenos Aires, and so Gerard flies to Argentina and is greeted at the airport by the portly Melchior Incza, who calls him Stephen Gerard and offers his services as a professional guide.  Replying that his name is not Stephen, Gerard rejects Incza's offer and registers at his hotel.

Gerard is looking up the phone numbers of Madeleine Jarnac and Tomas Camargo when Incza knocks on his door and invites him to a party at the Camargos' that night.  Intrigued, Gerard accepts, and at the party he meets Perchon, a Belguim banker; Tomas Carmargo, a wealthy industrialist and his wife; and Mme. Jarnac.  When Gerard introduces himself as a representative of her insurance company, Mme. Jarnac presents him to Ernest Dubois, the company's real representative.  Upon returning home that night, Mme. Jarnac finds Gerard waiting to question her.  After she insists that her husband was killed in 1943, Gerard condemns her for being a collaborator and begins following her.

Tailing her to a restaurant one day, Gerard is joined by Manuel Santana, the uncle of Tomas Camargo, who asks him to leave Mme. Jarnac alone.  As Gerard accuses Santana and Camargo of being fascists, Mme. Jarnac slips out of the restaurant.  Upon returning to his hotel room Gerard is met by Incza, who admits that he was hired to meet him at the airport and offers to take him to Mme. Jarnac.  Incza and Gerard drive to a convent just as Mme. Jarnac exists through a side door.  She agrees to meet him at his hotel lobby later that night, but when she fails to appear, Gerard goes to his room and finds Diego, a hotel valet, turning down his bed.  Diego hands him a sealed note from Mme. Jarnac, informing him that Jarnac is using the name Ernest Dubois and can be found at a house on Avenida Republica.

Proceeding to the address, Gerard breaks down the door and is about to shoot Dubois when he is struck from behind and knocked unconscious.  He awakens at Santana's house and finds Diego, Dubois and Santana there.  After examining the note, Diego announces that it was not written by Mme. Jarnac and discloses that they are working to unmask Jarnac and expose his connection to Camargo and his group of fascists.  Warning that only hard evidence and not vengeance can bring Jarnac to justice, Santana asks for Gerard's cooperation.

From Santana's house Gerard goes to see Mme. Jarnac, but when he finds that her rooms have been ransacked, he returns to his hotel and calls Incza.  After singeing a stack of blank papers, Gerard places the page that he found in the rubble on top and tells Incza that he has a dossier on Jarnac that implicates Camargo in his crimes.  Handing Incza the title page of the document, Gerard says that he plans to lock the rest in the hotel safe and instructs Incza to tell Camargo that he will expose his treachery unless he turns over Jarnac.

Later that night, Mme. Jarnac calls Gerard and instructs him to meet her at a train station.  As the trains rumble past, she admits that she has never met Jarnac and only agreed to marry him in exchange for money so that she could leave France with her sickly sister, a patient at the convent.  Mme. Jarnac begs for Gerard's help to escape Jarnac's web of intrigue, but he refuses to believe her story, and follows her to the hotel in which she is hiding.  When he returns to his hotel room, Gerard find Incza, who directs him to the Camargos' hotel suite.

While Mrs.  Camargo tries to seduce Gerard in her suite, Incza breaks into the hotel safe, seeking the dossier.  Discovering that it is not in the safe, Incza begins to search Gerard's room but his interrupted by Diego.  Incza demands that Diego produce the papers, and when the valet refuses, Incza shoots him.  Gerard is arrested for Diego's murder, but Santana has the charges dropped for lack of evidence.  Because his passport is not in order, however, Gerard is ordered to leave the country in forty-eight hours.  When Incza tells Gerard that Jarnac is in town and suggests that Mme. Jarnac might know where his office is, Gerard visits her.  After informing Gerard that her sister has died and that Santana has agreed to help her leave the country, Mme. Jarnac remembers the name of a waterfront café used by Jarnac and Camrago as a meeting place.

With time running out, Gerard goes to the café and is greeted by Camargo and Perchon, the Belgium banker.  As Jarnac begins to speak from the shadows, Perchon knocks Gerard unconscious.  Soon after Gerard regains consciousness, Incza arrives, and when he is unable to produce the dossier, Jarnac shoots him, obliterating his face with gunshots.  When Jarnac orders Camargo to identify the body of Incza as Jarnac and tell the police that Gerard and Jarnac killed each other, Camargo objects, and the two men begin to argue.  In the chaos, Gerard attacks Jarnac and begins to beat him and Camrago runs away.  As Gerard pummels Jarnac, Santana and Dubois arrive at the café and restrain him.  After pronouncing Jarnac dead, Santana believes that he has failed in his quest to expose the fascists until Gerard pulls a set of documents from Jarnac's pocket.  Mme. Jarnac then enters the room, and after Santana examines the documents, he announces that they contain proof that Jarnac controlled the industries owned by Camargo.  With the evidence against the collaborators in Santana's hands, Mme. Jarnac and Gerard have redeemed themselves, and Santana vows to exonerate Gerard of Jarnac's murder and convict the collaborators.