Ferrando and Guglielmo defend their ladies, the sisters Dorabella and Fiordiligi, against Don Alfonso’s attack on the ideal of women’s constancy. A wager of a hundred sovereigns is made, and Don Alfonso promises to prove the inconstancy of all women, including the sisters, if the two young men will put themselves unreservedly in his hands for twenty-four hours. They promise to do so, and amuse themselves by planning how to spend the money they are certain to win.

The scene changes and the sisters are discovered in sentimental raptures over the miniature portraits of their lovers. Their daydreaming is interrupted by Don Alfonso, who has sad news: Ferrando and Guglielmo have been called to war and must leave immediately. The young men enter, say their farewells, and march off to battle.

Disconsolate, the sisters are served breakfast by their maid, Despina. When Dorabella gives vent to her unhappiness, Despina advises that lovers’ absences are time for sport, not sadness. Disgusted, the sisters go out, and Don Alfonso enters to enlist the support of the resourceful Despina in the wager. The two young men appear in disguise as exotic foreigners and Despina and Alfonso conspire to have them win the affections of the ladies. The sisters are indignant at finding two strange men in their house, but Don Alfonso enters and pretends to recognize the impostors as old friends. He urges the sisters to be kind to the visitors. Fiordiligi makes it quite clear that their protestations of love are unwelcome, but the men persist until the ladies flee the room. Guglielmo and Ferrando are sure they have won their bet, but Don Alfonso reminds them of their promise to give him twenty-four hours to prove his point.

The ladies are in the garden when suddenly the strangers rush in with “poison” which Don Alfonso is unable to prevent their drinking. As the men sink into a feigned coma, a doctor is hastily summoned. The doctor (Despina in disguise) employs the latest medical devices and restores the youths to health. Ferrando and Guglielmo, thus restored, imagine that they are already in the Elysian Fields and demand a kiss from the Goddesses. This is vehemently denied them as the act ends.

Despina tries to persuade her mistresses that it is silly not to enjoy themselves while they have the opportunity. The ladies finally weaken, and, deciding to have some innocent fun, each selects one of the strangers as her partner: as it happens, the other’s fiancé. With Don Alfonso’s and Despina’s prompting they meet the men in the garden, and, after some embarrassed conversation, Fiordiligi leads Ferrando off to explore the garden. Guglielmo manages to exchange lockets with Dorabella, but Fiordiligi spurns Ferrando’s advances.

When the two young men meet to compare notes, Guglielmo is smug, but Ferrando is indignant and vows revenge. He resolves to try Fiordiligi’s defenses once more and interrupts her as she is about to put on a soldier’s uniform to join her lover at the front. Ferrando desperately implores her to run a sword through his heart. Her resistance suddenly melts and she falls into the disguised Ferrando’s arms. Immediate weddings for both couples are indicated, and as the girls go to prepare for the ceremony, the men comment on womankind: Così fan tutte – that’s the way all women are!

As the wedding contract is being prepared, a burst of martial music announces that Ferrando and Guglielmo have returned. The young men appear and accuse their ladies of inconstancy. But all is forgiven when the two men reveal that it was they who were behind the disguises. The couples are united and all agree a valuable lesson has been learned.