Dominique, a law student at the Sorbonne, is engaged to a fellow classmate. Unfortunately, she's more attracted to his philandering Uncle Luc, who's married to the charming Francoise. Dominique and Luc begin a tawdry affair...

In France, following the death of their son in a motorcycle race, the Vallons are plummeted into an unbearable state of sadness from which Madame Vallon never recovers. When the Vallons' daughter Dominique comes home on holiday from her studies at the Sorbonne, she finds her mother and father still paralyzed by grief. Upon returning to school, Dominique is warmly greeted by her boyfriend, Bertrand Griot, who takes her to a café to meet his worldly uncle, Luc Ferrand. When Bertrand asks for a ride to his mother's Sunday night dinner, Luc insists that they first join him and his wife Francoise for drinks. That Sunday, Dominique, nervous about meeting Bertrand's mother for the first time, develops a terrible headache and Francoise, sensitive to the young girl's anxieties, comforts her and gives her a beautiful red coat to wear. Over dinner, Bertrand's wealthy, overbearing mother refuses to lend her son the money he needs to marry Dominique, and after she sends Bertrand to fetch her a drink, Luc offers to give Dominique a tour of the gardens. There, he passionately kisses Dominique, who falls under his spell. Upon returning to her room, Dominique acts skittish and distant toward Bertrand, and then asks to be left alone. Obsessed with thoughts of Luc, Dominique breaks off her relationship with Bertrand. Concerned, Francoise visits Dominique to inquire if she quarreled with Bertrand, and when Dominique breaks down in tears, Francoise solicitously invites her to dine with some friends that evening. When Dominique becomes tipsy at dinner, Luc drives her home and then invites her to join him alone on the Riviera for one week that summer while his wife is visiting her mother. In a moral quandary, Dominique seeks out Bertrand and asks him to take care of her. After Dominique returns home for summer vacation, Luc sends Dominique a letter, telling her where to meet him in the South of France. Trying to protect herself from temptation, Dominique invites Bertrand to visit the Vallon house. When Bertrand arrives, however, his presence reminds Madame Vallon of her late son and she silently sobs and retreats to her bedroom, compelling Bertrand to leave. Furious at her mother's reaction, Dominique begs Bertrand to elope with her, but he level-headedly insists on waiting to get married and then departs. Alone and spiteful, Dominique goes to meet Luc. Once ensconced in their hideaway, Dominique determines to stay with him forever, but he views their week together as just another fling. On the last night of their trip, Luc asks Dominique to stay another week, but business obligations force him to return to Paris. Rather than go home, Dominique decides to follow Luc to Paris, where the unsuspecting Francoise invites her to dinner. After Francoise warmly greets Dominique, Bertrand unexpectedly drops in. Drunk and jealous, Bertrand realizes that Dominique is having an affair with Luc, and when he reproaches her, she defends herself by declaring that she is in love. Aware of his uncle's incorrigible womanizing, Bertrand warns Dominique that she has no future with Luc. Soon after, Luc phones Dominique and informs her that they must stop seeing each other because Francoise has discovered he has been unfaithful. Unable to stay away, Dominique begins to stalk Luc and spots him with another woman. When Dominique becomes jealously hysterical, Luc suggests she go home to her family. Instead, she runs down the street and is hit by a car. Finding Dominique unconscious in the road, Luc brings her home and Francoise realizes that they have been having an affair. Nevertheless, Francoise nurses Dominique back to health and then sends for her father to take her home. Feeling betrayed, Francoise acts coldly toward Dominique, but when Monsieur Vallon arrives, carrying the red coat, Francoise holds the coat for Dominique and then lovingly buttons it up. After Dominique departs, Francoise packs her suitcase and informs Luc that she is leaving him. Penitent, Luc begs her to stay and promises to be faithful. As a new term begins in the fall, Bertrand encounters Dominique in the school's foyer and then walks by her side to class.