A woman of easy virtue rejects a proposal of marriage made to her by a rich lover. But when her son is taken from her to be brought up in the best conditions, she keeps thinking about him. When she finds him, twenty years later, she can not adapt to the environment to which he now belongs.

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.