Chronic serial womanizer Stephane Margelle (Jean-Paul Belmondo) drops his wife Sophie (Marie Laforet) off at the airport so she can go away for Easter weekend. He immediately picks up beautiful young Julie (Sophie Marceau), who has just had a fight with her married boyfriend. He gets her back to his apartment and is preparing for a sexy weekend, when his wife suddenly returns home.
He makes up a bizarre, on-the-spot, spur-of-the-moment story that the gorgeous girl is actually his long-lost daughter. Julie plays along, but this leads to a whole series of increasingly ridiculous lies and comical situations (such as when her real mother shows up).