[Plot Summary]
In 18th-century Naples, two young officers, Guglielmo and Ferrando, proudly boast of their unwavering faith in the chastity of their fiancées, the sisters Fiordiligi and Dorabella. To this, the cynical old philosopher Don Alfonso proposes a wager, saying, "A woman's heart is like a reed."
Following Don Alfonso's plan, the two officers bid a false farewell, claiming they must suddenly depart for the battlefield. Soon after, they reappear in disguise as Albanian nobles with mustaches. They begin to seduce each other's fiancées, but initially face firm rejection from the sisters. In particular, Fiordiligi sings of her chastity being as solid as a rock ("Come scoglio") and repels their advances.
Determined to win the wager, Don Alfonso recruits the maid Despina to shake the sisters' resolve. As the courtship continues, the younger sister Dorabella opens her heart to the disguised Guglielmo, and Fiordiligi also succumbs to Ferrando's persistent seduction.
Ultimately, the sisters even sign new marriage vows with the disguised unknown nobles. At that moment, a military march is heard, announcing the return of their original fiancés. Bewildered, the two officers reveal their true identities and expose that everything was a theatrical deception. Though great confusion and self-recrimination ensue, Don Alfonso urges reconciliation, saying, "This is human nature." The four young people forgive one another and return to their original partners as the curtain falls.