Marcel, a simple-minded factory worker, is tricked into buying a high-priced American convertable car by a widow determined not to let it fall into the hands of her late husband's secretary/secret lover. Once in pocession of the car, Marcel only encounters one bad luck episode after another with the excessive gasoline consumtion, his wife trying to sell it to make ammends meet, getting into traffic jams, accidently riding into a car wash with the top down, and more.

Madame Lucanzas, furious because her late husband has willed his enormous American convertible to his "private secretary," sells the car to Marcel, a factory worker, for 500 francs ($100) in order to deprive the "secretary" of her inheritance. Being the owner of such an elegant car, dubbed "La Belle Américaine," causes Marcel innumerable difficulties. Gasoline consumption threatens to bankrupt him; his envious boss fires him; his wife Paulette turns a televised exhibit of dogs, fashions, and luxury cars into a shambles; the secretary locks Marcel in the trunk of the car when he refuses to sell it back to her; he mistakenly drives into an automatic car wash with the top down; and he becomes embroiled in a traffic jam involving official government cars. The last misfortune, however, proves to be a lucky break, for the Minister of Commerce becomes so impressed with "La Belle" that he strikes up a friendship with Marcel. Consequently, when the car is lost, the Minister uses his influence to see that it is promptly returned to Marcel. The problem of what to do with the car is finally solved when Paulette crashes it into an ice cream cart, and "La Belle Américaine" is converted into a palatial ice cream stand that creates a sensation at the race track.