Jean-Louis (Jean-Louis Trintignant) is a devout Roman Catholic who subscribes to an austere moral code influenced by the philosophy of Blaise Pascal. When he spots a pretty girl (Marie-Christine Barrault) at church, he promises to one day marry her. But then an old friend (Antoine Vitez) introduces him to alluring divorcee Maud (Françoise Fabian). After a conversation about love and philosophy, the chaste Jean-Louis spends the night at Maud's place, conflicted about what he desires.