A rich man's attempts to steal away a poor man's wife set the stage for this comic farce, set in Naples in 1860. Luca is a simple but honest man married who runs a mill and his married to Carmela (Sophia Loren), a strikingly beautiful woman.
Carmela's face and figure attracts the eye of the Governor (Vittorio De Sica), a Spanish nobleman who has been appointed ruler of Naples. Determined to have Carmela as his own, the Governor has Luca arrested on false charges, and sets out to seduce her while her husband is behind bars.
When Carmella resists, the Governor plays his trump card -- he is willing to free Luca, but only under the condition that she sleeps with him first.
Carmella is appalled, but dreams up a way to use the situation to her advantage; she slips the Governor a mickey, and after he's asleep, she makes haste to the prison, carrying the Governor's pardon of her husband. However, Carmella arrives to discover a surprise -- Luca has already escaped from the jail.
When Luca makes his way home, he discovers the Governor, still fast asleep, and is convinced he's already seduced Carmella; enraged, he sets off to the Governor's mansion, determined to get revenge with the Spaniard's wife. Bella Mugnaia was based on a novel previously filmed as Il Cappello a tre punte in 1934.