A hilariously inveterate womanizer (Jean-Paul Belmondo) drops his wife (Marie Laforet)off at the airport so she can go away for Easter weekend. He immediately picks up a young woman (Sophie Marceau), who has just had a fight with her married boyfriend. He gets her back to his apartment and is preparing for a long weekend of hot stranger sex, but then his wife suddenly returns, and he has to make up a spur-of-the-moment story of the young girl being his long-lost daughter. The girl plays along, but this leads to a whole series of increasingly ridiculous lies and comical situations (such as when her real mother shows up).
This is an old-fashioned European comedy that tends to differ from more modern Hollywood comedies in that the protagonist does not have to be sympathetic, but can be a philandering cad or hypocritical blow-hard. Belmondo though, a legend in French movies since his seminal appearance in Godard's "Breathless", does make his character quite charming, even though he's a chronically philandering cad who tells one bald-faced lie after another. Marie Laforet is also good as his wife, who keeps a good poker-face throughout, so you're never sure how much, if any, of his ridiculous stories she is actually buying, or whether she is just torturing him for her own amusement. Sophie Marceau shows off her spectacular, nubile breasts quite a bit (mostly just to tease her lecherous much older suitor), but she also easily goes toe-to-toe with Belmondo acting-wise (as she would the next year with Gerard Depardieu in "Police"), which is pretty impressive for an actress who (if IMDb dates can be believed) was only 17 or 18 at the time.
Continental European movies at that time really specialized in wild car chase scenes that would make Hal Needham hang his head in shame, so there are a number of those kind of shoe-horned into the plot as well. Still, all the zany situations, bare breasts, and car chase footage serve to speed this comedy along, making it a pretty fun ride even it's pretty much as light as a soufflé and about as substantial. I actually kind of miss comedies like this, compared to the ones today where the main character always has to be likable (even when is he is played by Adam Sandler) and invariably falls in love and/or learns a moral lesson by the end. I suppose these old-fashioned European movies could be considered more "sexist" (as opposed to, say, "Just Go with It" and "I Now Pronounce You Chuck and Larry"), but I don't personally buy that as even the young girl here is far more clever than the buffoonish male. This movie is mostly just a lot of harmless fun.