A mischievous nobleman and his poor doppelganger get into serious trouble when a practical joke ends badly and inadvertently ends up endangering the Pope.

Rome, Year of our Lord 1809. The Pope Pius VII with his cardinals and ministers manages both temporal and spiritual power of the Papal States in Italy. The Marquis Onofrio del Grillo is one of his favorites, but also the worst of all the nobility. As a privileged and protected nobleman, Onofrio feels free to play his pranks on the poor people without any fear of the consequences. On one occasion, when he is arrested at a dinner with common criminals, he turns to the populace in a vulgar speech, claiming that his nobility allows him to do what he wants, and that they, being poor, are not worth anything.

When a theater company arrives from Paris, Onofrio decides to start an affair with the beautiful and free spirited singer Olympia. One night, while walking around the ruins of the Forum to find a suitable spot for sleeping together, they notice a drunken coalman who is the very image of the Marquis. Onofrio decides to play one of his jokes by switching roles. He instructs his servant to have the unconscious man dressed up as himself, while he will play the part of Gasperino the coalman. The next morning the poor drunkard wakes up in Onofrio’s bed to find himself transformed into a marquis. His bad manners lead the family to believe that he is possessed by the spirit of a dead coalman, and Onofrio’s uncle tries to have him exorcised. After the first shock Gasperino starts to adapt to his new role and some of his family find him even better than the real Marquis. But when the Pope returns after Napoleon’s defeat, he has Onofrio condemned and Gasperino risks ending his life under the guillotine.