Bitru is a carefree fellow who, one day, decides to buy a houseboat on the River Seine. He moves there and starts living surrounded by a circle of colorful friends. Among them, the daughter of an auto maker but also a group of scientists who have developed "Aqua Simplex", a device that could enable motorists to fill their tanks with... water ! Panic sets in in the automobile industry and Bitru is appointed his friends' middle man to represent them on the various car makers directors' boards.