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.

Towards the end of the 19th century, the Duke of Gerolstein traveled to Paris. Sixteen years earlier he had a daughter who was taken from him by her mother who was chased out of the palace. After many dramatic adventures, the Duke finds his daughter in the person of Fleur de Marie, martyred throughout her childhood by the Owl and the schoolmaster.