Based on the play by Pierre Wolff.

After a New Year's Eve party, 30-year-old fashion designer Rebecca (Nathalie Schmidt) awakens alone in a strange automobile. Later, during a routine visit to her gynecologist, she learns that she's pregnant. Unable to recall anyone responsible, she attempts to find the father as she treks through Paris streets, quizzes her girlfriends, and visits a Belgian detective. With few leads, her search drags on for months.

Secret de Polichinelle roughly translates as Open Secret. The "secret" in question is an illegitimate child, the offspring of young-and-foolish Henri (Bernard Lacret). The baby is adopted by its grandparents, Monsieur and Madame Jouvenol (Raimu and Francoise Rosay). At first taking charge of the child because it is their duty, the Jouvenols come to love the little nipper as if he were their own son. At this point, the film threatens to drown in a morass of sentiment, but the actors and the director manage to stem the bathos with some first-rate comedy vignettes revolving around the care and feeding of the bouncing baby boy.