Seated in a cafe, Ferrando and Guglielmo defend their ladies, the sisters Fiordiligi and Dorabella against an attack on women by the cynical Don Alfonso. Don Alfonso promises to prove the inconstancy of 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 sure to win.

The scene changes and the sisters are discovered in sentimental raptures over the miniature portraits of their lovers. Their day-dreaming 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 sad farewells, and march off to a military air. The girls pray for their safety.

Disconsolate, the ladies are served chocolate by their maid, Despina. When Dorabella gives vent to her unhappiness, Despina advises another line: lovers’ absences are time for sport, not sadness. Disgusted, the sisters go out, and Don Alfonso enters to enlist the support of the clever Despina. The two young men appear in disguise and Despina and Alfonso plan to have them win the affections of the ladies in the guise of strangers. The sisters are indignant at finding two strange men in their house, but Don Alfonso enters and pretends to recognize the imposters as old friends. He urges the sisters to be kind to the visitors. Fiordiligi makes it quite clear that their protestations of love are fruitless, 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 walking in the garden when suddenly the masqueraders rush in with bottles of poison which Alfonso is unable to prevent their drinking. As the men sink into a coma, the 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 denied them as the act ends.

Despina is discovered in the process of pointing out to her mistresses that it is silly not to enjoy themselves while they can. The ladies finally weaken, and deciding to have some innocent fun, each selects one of the strangers as her partner. Under 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 effusively 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 their bridal dresses, the men comment on womankind: Così fan tutte – so are they all.

The the wedding contract is prepared and signed in the presence of the notary (again a disguised Despina) and all is ready for the newlyweds to leave, when a burst of martial music is heard. Alfonso announces that Ferrando and Guglielmo have returned. The Albanians are rushed out and  the young men appear. They discover the marriage contracts, and accuse their ladies of inconstancy. But all is forgiven when the two men reveal that it was they who were behind the disguises, and the two couples vow eternal fidelity.