Jean-Louis Jean-Louis Trintignant, a solitary, serious Catholic engineer recently relocated by Michelin to Clermont-Ferrand, is certain he will marry a young blonde woman named Françoise Marie-Christine Barrault whom he has seen at church. At a bar, he runs into his old school-friend Vidal Antoine Vitez, an atheist Marxist who is now a philosophy professor. They discuss Pascal, and Vidal invites Jean-Louis to a performance by violinist Léonide Kogan that evening. Afterward, Vidal asks if they can get together the next day, which is Christmas Eve, but Jean-Louis is going to Midnight Mass. Vidal accepts an invitation to come along, and arranges for them to visit his friend Maud Françoise Fabian afterward, though the visit gets delayed a day, as she has to see her ex-husband, with whom she has a daughter. When Vidal mentions he and Maud were lovers, Jean-Louis offers to let Vidal go alone, but Vidal says they are incompatible as a couple, and he wants Jean-Louis to help make sure the visit remains platonic.
At Maud's apartment, Jean-Louis, Vidal, and the brunette, atheist pediatrician have a discussion about religion, Pascal, and Jean-Louis' relationship history. When it starts to snow, Maud, worried the drive to Jean-Louis' mountain village will be unsafe, offers her guest room. Vidal encourages Jean-Louis to stay and leaves.
Maud makes herself comfortable on her living-room bed. She mentions she and her husband both had affairs: he with a Catholic woman whom she despised, and she with a man who died in a car crash on icy roads. When Maud reveals there is no guest room and invites Jean-Louis to join her in bed, a shocked Jean-Louis fails to get comfortable in a chair before deciding to sleep under a blanket on top of Maude's bedspread. Early in the morning, they kiss, but Jean-Louis pulls away. Maud recovers quickly and, as he prepares to go, reminds him of a day trip to the mountains with Vidal and some others that afternoon.
On his way to meet up with Maud and her friends, Jean-Louis sees Françoise. He chases her down, and they arrange to have lunch after Mass the following day. In the mountains, Jean-Louis and Maud kiss, and she teases that she is not right for him, as she is neither Catholic, nor blonde. Back in town, they go shopping and make dinner together, and, before leaving, Jean-Louis says he thinks he has so quickly come to feel so happy around Maud because, as she is moving to Toulouse soon, "The thought of the future needn't depress us, since we have none." They part, smiling.
Jean-Louis sees Françoise and offers to drive her home. She is a biology postgraduate who also works at a lab, and, on the turnoff to her house, Jean-Louis gets stuck in the snow. Françoise offers to let him stay overnight in the room of one of her housemates, who all went home for the holiday, and they talk about relationships and choices over tea before retiring.
Before leaving for church in the morning, Françoise gently rejects Jean-Louis' attempt to kiss her. He says he loves her, but she says he does not know her, and might disappoint him. They begin to date, and Françoise admits that, until recently, she was having an affair with a married man, who, although she loves Jean-Louis, she has not forgotten. He says they can move slowly, and that he still loves her, and is even glad, as he felt guilty for having past affairs, but now they are even. Françoise asks that they never again discuss this subject.
On holiday five years later, Jean-Louis, now married to Françoise and with a son, sees Maud at the beach. Françoise and Maud, who know each other, exchange greetings as Françoise passes. Maud and Jean-Louis briefly reminisce, and she mentions she is in another unhappy marriage. Afterwards, Jean-Louis tells Françoise that Maud is the woman who he mentioned having spent the night with just before they met, and is about to clarify that they did not have sex, when suddenly he realizes Françoise was the mistress of Maud's husband, and she is nervous Maud may have mentioned this. He does not broach the topic, she smiles, and they take their boy for a swim.
