Young Florville at last musters up enough courage to ask Gaudenzio’s pupil Sofia to marry him. With the help of the maidservant Marianna he arranges a secret meeting with his beloved, but she informs him that her guardian intends to give her in marriage to a wealthy suitor, the son of a certain Bruschino, whom no one has as yet set eyes on. Filiberto the innkeeper, who is owed a substantial sum by the drinking and gambling son of Bruschino,  turns up, announcing that the young man will remain his prisoner until he will have paid his debts. He also has a signed letter in which the young Bruschino begs Gaudenzio to plead to his father for his liberation. Taking advantage of the situation, Florville declares that he too is called Bruschino and is cousin to the man of the same name. He takes the letter, pays half of the money owed, and recommends that the debtor be further detained until his entire debt has been settled.

Florville has no difficulty in persuading Gaudenzio that he is the young Bruschino until the gout-ridden father Bruschino appears, griping and grumbling. Faced with the man who claims to be his son, he is thunderstruck and doesn’t recognize him. Gaudenzio accuses him of being a degenerate father without pity for the innocent foibles of youth and Bruschino cannot convince his friend that he has never seen this chap. To clarify the situation, they call the police. The officer has also received a letter from young Bruschino, asking him to intercede with his father; comparison of its handwriting with that of the letter obtained by Florville from Filiberto reveals two identical signatures. What’s more, Filiberto turns up in search of the money he is owed, calling Florville ‘Bruschino’, since he had claimed to be a member of that family. The police officer therefore forces Bruschino to acknowledge Florville as his son. The poor father teeters on the verge of madness.

Gaudenzio thinks that Bruschino is trying to wriggle out of his agreement to give his son in marriage to Sofia. Furious, he tries to precipitate the wedding ceremony. As for Bruschino senior, he discovers that the unknown young man is the son of the famous senator Florville, Gaudenzio’s sworn enemy. In order to play a trick on the latter, he pretends to recognize his son at last and the wedding immediately takes place.

At that very moment the real son of Bruschino, announced by Filiberto, appears, asking for his debauched conduct to be forgiven. A petrified Gaudenzio has to acknowledge the new situation and all join in the triumphant song celebrating the power of love.