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.

Variations fantaisistes sur les thèmes "l'argent ne fait pas le bonheur", "les grandes inventions pourrissent l'humanité", exécutées par une bande d'aimables farfelus guidés par le sarcastique et mystérieux Bitru. Finalement, les moteurs marchent à l'eau claire, les financiers sont perplexes, l'amour récompensé et toute la troupe s'embarque sur la péniche de l'évasion.