In an Oil Company, due to a confusion, César Dandieu, honest cash clerk and Fernand Mouchette, freakish inventor, change their suitcases, same models. So that, at the end of the day-work César gives to his Manager the case pertaining to Fernand. This suit-case contains a prototype of a new carburettor which was to be proposed to the Oil-Company, instead of the cash money that César was supposed to bring. He is sent to jail, although not guilty. In the meantime, Fernand has brought César's case at home, where he has not even time to open it, being caught by gendarmes (constables) for late military service. In his flat, his charming fiancée, Adèle, leads, as well as him, a Bohemian life : she doesn't care neither for the suit-case, nor open it. Back from jail for César and from military service for Fernand, they meet by accident, become friends, and decide a revenge upon Human Society. Being dishonest, they become very rich.
Source : IMDb