Don Alfonso has raised doubts about whether the fiancées of Ferrando and Guglielmo are as faithful and virtuous as they believe. The two men defend their brides-to-be, the sisters Dorabella and Fiordiligi. Don Alfonso suggests a wager in which the faithfulness of the two ladies would be put to the test. Guglielmo and Ferrando agree, and all three look forward to celebrating the outcome of the bet.
Fiordiligi and Dorabella blissfully contemplate miniature portraits of their lovers. Don Alfonso arrives and breaks the news that Guglielmo and Ferrando have been summoned to serve their country; the two men enter and bid farewell to their fiancées. As arranged by Don Alfonso, a troop of soldiers arrives to collect Guglielmo and Ferrando. The two sisters and Don Alfonso wish them smooth sailing on their journey.
The sisters' personal assistant, Despina, grumbles about the injustice of her subservient position as she prepares refreshments for the ladies. Fiordiligi and Dorabella give way to their overwhelming grief but encounter unexpected advice from Despina, who tries to teach them that men are worthless and replacable. Don Alfonso enlists Despina as an accomplice in his plan to console the sisters in their lovers' absence by introducing them to two handsome, exotic visitors.
Disguised, the two men enter: neither sister recognizes them as Guglielmo and Ferrando. The men's initial attempts at seduction are met with firm rejection; Fiordiligi and Dorabella depart in anger. The two men consider their wager already won, but Don Alfonso reminds them they agreed to do his bidding until the following morning.
Later that day, the men stage a fake suicide by poison to excite the sympathy of Fiordiligi and Dorabella. The sisters call for Despina, who runs off with Don Alfonso to fetch a doctor. Despina returns in a doctor's disguise and “cures” the stricken men. Though now interested, Fiordiligi and Dorabella refuse to kiss the strangers to seal the joy of their recovery.
Despina encourages Fiordiligi and Dorabella to welcome the attentions of their new suitors. The sisters agree to join the game in order not to die of boredom while their true loves are away. Don Alfonso and the two imposters have arranged a serenade for the ladies. Guglielmo and Ferrando, in disguise, set about seducing each other's fiancée. Fiordiligi pairs off with Ferrando, Dorabella with Guglielmo. Dorabella yields to Guglielmo, accepting a token from him and giving up her miniature portrait of Ferrando. Meanwhile, Fiordiligi tells Ferrando to leave her alone; but when he has done so, she confesses that the stranger has aroused her passions, and she begs her absent fiancé's forgiveness. When the ladies have left, Guglielmo reveals to Ferrando that Dorabella has been unfaithful. As Ferrando rages, Guglielmo pontificates on the fickleness of women.
Despina congratulates Dorabella, who counsels Fiordiligi to give in to love as she has done. Fiordiligi decides that only by joining their lovers will they be safe from temptation, and she prepares to depart. But Ferrando returns and this time Fiordiligi is incapable of resisting. Now it is Guglielmo's turn to rage. Don Alfonso suggests that marriage is the only solution, and Despina leaves to find a notary to draw up the marriage contract.
The notary (Despina in disguise) enters, and the ladies sign the contract. Drums sound in the distance, and Don Alfonso reports that Guglielmo and Ferrando have returned from war. The suitors and Despina run and hide. Guglielmo and Ferrando return as themselves, and pretend to be horrified to learn that their fiancées have married other men in their absence. The women admit their shame and remorse, but point to Don Alfonso and Despina as the instigators. Guglielmo and Ferrando go after the false suitors and reemerge, revealing the deception.