A house near Naples. Two sisters, Fiordiligi and Dorabella, and their lovers, Guglielmo and Ferrando, have been arguing with an older couple, Don Alfonso and Despina, about the possibility or impossibility of love and fidelity. In spite of their past experiences, Ferrando and Guglielmo have faith in the constancy of their brides-to-be. Don Alfonso challenges them to a wager on the faithfulness of their partners: In the space of a day, he will prove to them once and for all that women are incapable of being true. The young couples agree to do whatever Don Alfonso asks of them for the next twenty-four hours. Alfonso engineers a separation: Guglielmo and Ferrando pretend that they are going off to war, and the couples experience the pain of parting. Vowing to be true, with Alfonso and the women wishing them a safe voyage, the men leave.
Despina curses her existence as a servant. Dorabella feels as though she will die of sorrow. For Despina, the fact that the men are gone is a positive: Since the women were merely an amusement to the men, now the women should amuse themselves. Alfonso gives Despina some money to secure her cooperation with his scheme for the young couples.
Ferrando and Guglielmo return in disguise, and Alfonso introduces them as his Albanian friends. The women mock the young men’s clothing and ask that they be thrown out of the house. As foreigners, the young men court their fiancées – at first to no avail, with Fiordiligi comparing her fidelity to a rock. The men try to win the ladies over with an exhibition of their manliness. When the women show no interest, Ferrando and Guglielmo feel confident of their victory over Alfonso.
But with Despina’s help, Alfonso comes up with a new stratagem: The young men pretend that their lovesickness has driven them to take poison. Disguised as a doctor, Despina revives them. Caught up in the game, Dorabella and Fiordiligi draw closer to the two “victims” – until the men ask for a kiss, which the women angrily refuse.
Despina marvels at the women’s prudish behavior. Advising them to treat love lightly, she teaches them the art of seduction. Dorabella encourages Fiordiligi to follow Despina’s advice. They decide to accept the disguised men’s overtures. Dorabella chooses “the brown-haired,” Fiordiligi “the blond one.”
Alfonso shows the men how to reconcile with the ladies, successfully getting himself back into Despina’s good graces with the help of flowers and repentant words. Ferrando and Fiordiligi go off by themselves. Guglielmo feigns another collapse, claiming to have been poisoned by Dorabella’s amorous eyes. He offers her his heart – and steals, in return, her locket, with Ferrando’s portrait inside it. Fiordiligi manages to elude Ferrando’s advances once more. Wrestling with her feelings, she begs her lover’s forgiveness. Ferrando is convinced again that he and Guglielmo will win their bet with Don Alfonso, but then his friend reveals the token of Ferrando’s love that he has won from Dorabella. Guglielmo reviles women for their fickleness; Ferrando feels betrayed on all sides.
Alfonso demands that the two men let their game play out for its allotted time. Despina encourages the women to trust their feelings and keep playing along: Dorabella wants to follow amore, while Fiordiligi continues to struggle with her emotions. Now it is Guglielmo’s turn to witness an amatory dialogue between Fiordiligi and Ferrando. Alfonso consoles the young men with the cynical message that all women act the same way.
Despina suggests they conclude the game with a wedding ceremony; she herself will play the notary. Showered with well-wishes, the young lovers drink to beauty and forgetting. Once the marriage contract has been signed, Don Alfonso announces the return of the departed fiancés. Amid heated accusations and counteraccusations, all the masks come off, and the assembled group sings the praises of common sense.