On the Indian Ocean island of Réunion, Louis Mahé, a wealthy tobacco plantation owner, awaits the arrival of his bride-to-be, Julie Roussel, whom he has never met. They had connected through the personals column of a French newspaper, exchanging letters that promised a life of luxury and companionship. At the Hotel Mascarin, he meets his business partner Jardine, who accompanies him to pick up the engagement ring. Louis drives to the dock to greet [Julie Roussel], who is stepping off the steamer Mississipi from Nouméa, the capital of New Caledonia. When they finally meet, he is struck by her beauty and does not recognize her at first—the woman before him is not the image she sent in the photo. She explains that she sent a photo of a neighbor to prove her sincerity. He confesses that he has not told the whole truth either, having kept the fact that he is very wealthy.
They marry quickly, and his ardor makes him overlook inconsistencies between her remarks and her letters. He grants Julie access to his bank accounts and even prints her image on the cigarette packs produced by his company. After a stern letter from Julie's sister, Berthe, demanding Julie's whereabouts, Louis returns home to find that his wife has vanished with nearly 28 million francs, draining most of his savings. Berthe arrives soon after and reveals that his wife was not Julie and that she saw her sister board the Mississipi. They hire private detective Comolli to track down the impostor.
On a flight to Nice, Louis collapses from exhaustion. While recuperating at Clinique Heurtebise, he sees the impostor on television, dancing at a nightclub in Antibes. He buys a gun and travels to Antibes, breaking into her room at the Hotel Monorail, intending to kill her. When she returns, she offers no resistance. She reveals that her real name is Marion Vergano and recounts a sordid past—years in prison and an association with a ruthless gangster, Richard, who accompanied her on the Mississipi. She explains that, upon learning of Julie Roussel's forthcoming marriage, Richard concocted a plan to kill Julie and substitute Marion to rob Louis; he then forced Marion to go through with the robbery and abandoned her. She tells Louis that she loves him, and he forgives her.
They buy a convertible and drive to Aix-en-Provence, moving into a house as they roam the region and make love. Their happiness is interrupted when the trail of the impostor leads to Aix, and Comolli arrives on the scene in pursuit of the truth. Louis tries to bribe the detective to drop the case but fails, and the two men cross paths in a confrontation that ends with Comolli dead, buried in the wine cellar. The lovers flee to Lyon, but Marion grows restless with life on the run and longs for Parisian luxury. Louis returns briefly to Réunion and sells his share in the plantation to Jardine. Upon his return, the police close in again, forcing the couple to vanish into the mountains.
In a remote cabin, they hope to cross into Switzerland, but Marion's mood sours with the lack of stability. Louis grows increasingly ill and suspects Marion is poisoning him. He tries to escape, but she restrains him. As she pours him another cup of coffee, he reveals that he knows her plan and accepts his fate, expressing a complicated, reluctant love. Ashamed, Marion shatters the glass and vows to make amends. She finally confesses that no woman deserves to be so loved, yet she loves him in return and proposes they run away together. In a tearful moment, she whispers the film’s haunting line: > I'm learning what love is, Louis. It's painful. They set off into a snowstorm, heading toward the border, and into an uncertain future.
