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.