The historical Marquis of Grillo lived at the beginning of the 18th century in Rome. He was a colourful rake, a member of a still existing and well-known family, and the tales that have been attributed to him over the years have blurred the line between fact and fiction. Mario Monicelli transfers the action on to 1810, a time when Rome was occupied by Napoleon's troops, and the authentic atmosphere he creates in the film is one of its strongest elements.

Marquis Onofriodel Grillo, noble guard and Private Valet to Pope Pius VII, is a typical representative of the Rome aristocracy, whose only escape from the boredom of his palace are his excursions into the world of the lower classes.

His love of practical jokes makes him stage a special surprise for his family: One night he discovers his perfect double in a drunkard named Gasparino. He installs him in the palace as his substitute, where the poor fool wakes up to an inexplicable dream, but soon grasps his true role and tries to make the most of it.