A club of young men who live by logic find themselves in love, and their lives fall apart as they try to cope with the emotions their romance strikes on them.
The film begins in 18th century France, when a school is closed for the summer vacation. A band of four students and friends stay behind, who form a band called the Rational Party; its members are the handsome and clever Marion, the aloof and sly Lindo Hall, the cheeky but good-natured Jack Sydow and the quiet, devoted Claude. Marion is held as the ideal boy to love by many girls, but he stays rationally aloof and refuses to emotionally involve himself with anyone. But his logical aloofness is in fact a front - his mother keeps marrying new husbands and thus neglects Marion, so he copes by suppressing his emotions with rationalism.
At a café, the group observes two seniors fight over the beautiful Ledania, the mayor's daughter. Marion stops their fight and orders them to pay for the damages theyve caused; one of them is insulted and strikes Marion. In response, Marion challenges the senior to a duel: they stand on railroad tracks as a train approaches, and the first one to jump off the tracks loses. Marion wins by a split-second. His bravery and coolness attracts the attention of a passenger on the train, a beautiful woman named Sara Vieda, who gives Marion a complimentary kiss. This act of desire shakes and confuses Marion.
Ledania, who is attracted to Marion, writes to him to arrange a meeting. On a rainy day, they meet at a café and she confesses her feelings, but he rejects her in a refusal to love anyone at all, and she slaps him. Overcome, he runs away, but running in the rain he becomes ill and collapses. He is found by Sara and taken to her home, where she tends to him. He initially rails at being imprisoned against his will, but is too sick to resist Sara's advances. She seduces him, and the act opens his eyes to a world of emotion and passion.
A week passes. Claude is mad with worry at Marion's whereabouts, but behind his back Lindo and Jack know where he is. They decide to head to Sara's home to try and bring him back, but they discover he is a much changed individual: more mature, emotionally secure, and open to affection. Claude fumes at this change in his friend, but Jack is pleased at his friend having an improved demeanor. Ledania meanwhile keeps a dignified and disapproving silence and distance. And Lindo, who has his eye on Ledania, is making his own plans to gain her...
Lindo learns that Sara's patron, the Count de Cluny, will be arriving at her house in a month's time. He and Claude arrive at the station to inform the Count, but Claude backs out at the last minute, refusing to hurt Marion. Lindo informs the Count, but when the two of them meet Sara about this he is shocked to see them take it in stride and greet each other calmly and affectionately; Sara has had many other lovers, of whom Marion was just the latest, and she quietly decides her liaison with him over and done with.
Unaware of all this, Marion decides to visit Sara by horseback. In the stable, he finds Claude and an empty bottle of pills. Claude mutters that he wants to end his life, and then in a moment of passion and despair he confesses that he is in love with Marion. He throws himself on Marion and tries to ravish him, but a shocked and revolted Marion strikes him; unable to bear the rejection, Claude commits suicide by jumping into a river with slit wrists.
Regretting the way he treated Claude, Marion rushes to Sara's home for comfort. Ledania tries to follow him, but Lindo says he is not worth the kindness. An upset Jack reveals Claude had informed him of Lindo's informing on the Count, and accuses him of upstaging Marion to make himself look good to Ledania. Then he himself proposes to Ledania... Marion arrives, and discovers Sara with the Count. Then Ledania also arrives, looking for Marion; she informs him that Jack and Lindo's argument got carried away and the two are dueling each other with pistols. Marion rushes off to stop the duel, but as he rushes between them they fire at each other, both hitting him...
At the end of the summer vacation, Jack is the only one still at the school. Marion was badly wounded and returned to his mother in Marseilles (whether he recovered is unknown); Lindo left for another school; and Ledania still pines for Marion. That one summer has changed all of their lives forever.
