Modeste is a timid watchman at an automobile factory which is run with an iron hand by its owner, Roger Drapeau. One evening, Modeste leaves his post to visit a fairground, where he meets the attractive Aline. Having glimpsed his employer, Modeste makes a hasty return to the factory. When Monsieur Drapeau appears, Modeste explains that it must have been his twin brother that his boss saw at the fairground, a nasty piece of work named Raphaël. Not only does Drapeau fail to see through this unconvincing lie, but it gives him an idea to win a marathon car race. Modeste and Raphaël will take alternate turns in the stages of the race and, because they are so similar, no one will be any the wiser. Unable to admit that he has no brother but afraid of losing his job, Modeste allows himself to be coerced into taking part in the race...

Modeste est veilleur de nuit à l'usine du constructeur d'automobile Roger Drapeau. Un soir, son cousin Roméo, l'emmène dans un parc d'attractions pour lui présenter Aline qui travaille dans la baraque de sa tante, Mme de Vanves. Mais Roger Drapeau vient à passer par là, et Modeste, dès le lendemain, invente à son patron qui veut le renvoyer, une histoire de frères jumeaux. Mais bientôt, le malheureux employé va être contraint de tenir le rôle de cet imaginaire mauvais garçon : Raphaël le Tatoué.