Mathias Pascal, saddled with a stupid wife and a nagging mother-in-law, leaves home and is extremely lucky at several gambling resorts. He returns home and discovers that a drowned man, fished out of the river, bears an uncanny likeness to him and is being buried by his family as him. This, to him, is a pleasant turn of events and he goes to Rome, where he falls in love with Louise Paleari. Count Papiano, a jealous suitor of Louise's, threatens him with arrest unless he produces credentials to prove his identity.

A series of crimes is being perpetrated around a famous singer. Strangely, during each of the crimes, the artist's voice is heard humming his favorite song. Suspicions lead to a chase through a nocturnal Paris, ending in a disused theater where the assassin has lured his final victim.

The wealthy industrialist Alain de Narbois has disappeared. Michel Dunant, an idle man who passed by and who looks exactly like the deceased, is taken for Alain by a young woman and her servant. He jumps at the chance but quickly realizes that, in this hushed little world, masters and servants play comedy and know very well that he is only a double of Alain. In fact, the industrialist was assassinated.

Arlette thinks she has made a wonderful gift to her daughter Sylvie, who has just married François Morel, a young journalist. And, to tell the truth, not everybody receives a posh little bar like the Royal Montmartre as a gift. The trouble is that Monsieur Jo, the former owner of the joint is also the boss of a gang of drug traffickers. And that he has managed to persuade the pair of naive lovebirds to drop a painting to a friend on their way to their honeymoon place. Now the picture is stuffed with drugs! The honeymoon trip promises to be eventful...