Truffaut described the book as "a perfect hymn to love and perhaps to life" . He came across it during the mid 1950s whilst browsing through some secondhand books in Paris and later befriended the elderly author who approved of the young director's attempt to translate the words on the page into celluloid images.
The film is set before, during and after the First World War in several different parts of France, Austria, and Germany. Jules (Oskar Werner) is a shy writer from Austria who forges a friendship with the more extroverted Jim (Henri Serre). They share an interest in the world of the arts and the Bohemian lifestyle. Early in the movie, they become entranced with a statue of a goddess, smiling serenely.
After encounters with several women, they meet the free-spirited, capricious Catherine (Jeanne Moreau), a dead-ringer for the statue with the serene smile. Although she begins a relationship with Jules, both men are affected by her presence and her attitude toward life. A few days before the declaration of war, Jules and Catherine move to Austria to get married. The men both serve during the war; however, they serve on opposite sides and each fears throughout the conflict that he might have killed the other.
After the separation that occurs during the war, Jim visits, and later stays with, Jules and Catherine in Austria. Jules and Catherine have a little daughter, Sabine, but the marriage is a miserable one. Catherine torments and punishes Jules with numerous affairs, and once left him and their daughter for six months. She flirts with and attempts to seduce Jim, who has never forgotten her. Jules, desperate that Catherine not leave him forever, gives his blessing for Jim to marry Catherine so that he may continue to visit them and see her. For a while, the four of them live happily together in the same chalet in Austria, until tensions between Jim and Catherine arise because of their inability to have a child. Jim leaves Catherine and returns to Paris, and after several exchanges of letters, Catherine breaks off their relationship. Jim moves in with his lover, Gilberte, in Paris, yet returns to the chalet once more to try to have a child with Catherine. However, the couple is not successful in their attempts to conceive. Jim moves back to Paris another time, and receives word from Catherine that she is pregnant. Jim does not believe her and writes her that he cannot come to the chalet because he has a grave illness that keeps him in bed. Eventually, Catherine claims that her child has died "early in the womb" and Jim's and Catherine's relationship is now over.
After a time, Jim runs into Jules in Paris. He finds that Jules and Catherine have returned to France. Catherine attempts to win Jim back, but he rebuffs her, saying he is going to marry Gilberte. Furious, she pulls a gun on him, but he wrestles it away and flees. He later encounters Jules and Catherine in a very famous (at that time) movie theater, the Studio des Ursulines.
The three of them visit a park, and after lunch, Catherine suggests that Jim get into her car because she has something to show him. After telling Jules to watch them, she proceeds to drive Jim and herself off a bridge. Jules is left to dispose of the ashes of his friends.
The evocative musical score is by Georges Delerue. One song, "Le Tourbillon" (The Whirlwind), summed up the turbulence of the lives of the three main characters, becoming a popular hit. The dialogue is predominantly in French, with occasional lines in English and German.