Stéphane Petitbourgeois is having trouble with his son-in-law Hubert, who has an extra child every year and is expecting his ninth, making thirteen people in the same apartment. However, a competition promises twenty million to the youngest grandfather of twelve grandchildren. Stéphane begins to encourage Hubert. But Pat became sterile from that day on. Finally, after two years, a child is announced. In fact, triplets were born, and after a few twists and turns, Stéphane became the youngest grandfather of twelve grandchildren.
| Actor | Role |
|---|---|
| JJane SourzaIsabelle Petitbourgeois, la femme | Isabelle Petitbourgeois, la femme |
 Raymond SouplexStéphane Petitbourgeois, le mari expert-comptable | Stéphane Petitbourgeois, le mari expert-comptable |
 Louis de FunèsCélestin Ratier, représentant des industries de l'enfance | Célestin Ratier, représentant des industries de l'enfance |
 Jean CarmetHubert, le beau-fils | Hubert, le beau-fils |
 Andrée ServilangesGeneviève, dite Mademoiselle, la nurse | Geneviève, dite Mademoiselle, la nurse |
 Andréa ParisyPat, la fille | Pat, la fille |
 Marthe AlyciaDaphné, la mère d'Hubert | Daphné, la mère d'Hubert |
| FFlorence Blotl'employée de l'état civil | l'employée de l'état civil |
 Saint-Granierlui-même | lui-même |