In the train that takes her back home, Catheine Beryl meets concert pianist André Fuger, a childhood friend of her husband's. The musician is both good-looking and seductive and Catherine soon falls for his charms. The two young people spend the evening and the following night together. The next day, a lost powder compact gives the affair away. At a loss, Catherine runs away from her husband but she has an accident. When she regains consciousness in her hospital bed, Catherine finds Georges, her husband, by her side, pretending nothing has happened.

Catherine, la femme de Georges Berry, rentre de voyages 24 heures plus tôt que prévu, dans l'intention de rejoindre son mari le plus vite possible. Or, dans le train, elle rencontre un ami de Georges, avec qui elle passe la soirée et la nuit suivante. Un poudrier oublié révélera l'escapade...