Clémentine and Sébastien, young, overwhelmed parents, stretched between their work and children, find themselves further burdened with Marie-France, Sébastien's mother, who comes to stay with them after she discovers that her husband Henri has been cheating on her and is now expecting a baby with his mistress.

Distraught, Marie-France behaves like a teenager in a crisis, undermining the young couple's authority and morale. The birth of Sébastien's little brother and the blissful euphoria this later-life paternity provokes in his father is the final straw: everyone is left reeling and the order of generations is turned upside down.