Ferrando and Guglielmo, two young officers, declare that the two young women they love, Dorabella and Fiordiligi, could never stray in their affections. Their friend Don Alfonso, a philosopher, thinks otherwise – all women are alike – and wagers he can prove the point if his friends obey his orders for twenty-four hours. They accept the wager.

Fiordiligi and Dorabella, sisters from Ferrara, gaze fondly at portraits of their beloveds while waiting for them to arrive. Don Alfonso precedes them – with the news that the officers have been called to active service. Tearful farewells follow, and in a trio Fiordiligi, Dorabella and Don Alfonso pray that the pair may have a safe journey.

Despina, the sisters’ chambermaid, prepares their breakfast chocolate. They rush on in wildest despair; Dorabella declares that in her frenzy she will outdo Eumenides. When Despina learns what is wrong, she advises them, since their lovers are away, to acquire replacements quickly; no man is worth waiting for. They leave. Don Alfonso returns, summons Despina and enlists her support in a scheme to provide the sisters with new lovers. The prospective suitors appear: “Albanians” whose whiskers move Despina to mirth. The sisters return, indignant at finding strange men in the house, and affronted when the strangers break into protestations of love. In the grand manner, Fiordiligi proclaims her unswerving devotion to Guglielmo. The disguised Guglielmo, secretly delighted, responds with a catalogue of his and his friend’s merits. The women sweep out. Guglielmo and Ferrando burst out laughing; Don Alfonso advises them to see who has the last laugh: they are still under orders. Guglielmo wonders when they are to eat; Ferrando replies that a breath of love is all the sustenance a lover needs.

In the garden, Fiordiligi and Dorabella lament their fate. The “Albanians” rush on in feigned despair and drain little vials labeled arsenic. Don Alfonso advises the sisters, while he and Despina go to find a doctor, to comfort their last moments. A doctor – Despina in disguise – appears and, wielding a mighty magnet, restores the men to life. They ardently entreat an embrace. The act ends with the sisters’ angry refusal, the officers’ hope that anger will not turn to love and Don Alfonso’s and Despina’s confidence that it will.

Despina advises her mistresses to be women of the world and accept the “Albanians.” They, not without misgivings, decide that perhaps they will – or at any rate give it a try. Dorabella chooses the handsome dark one. Don Alfonso summons them to the garden.

There, a serenade awaits them. But all four lovers prove tongue-tied, and so Don Alfonso and Despina speak for them. At last, in pairs, they stroll through the garden. Guglielmo induces Dorabella to exchange her portrait of Ferrando for a locket he gives her. Ferrando’s wooing of Fiordiligi is less successful: she dismisses him and reproaches herself at some length for having, even for a moment, forgotten her Guglielmo. When the men meet again, Ferrando tells a delighted Guglielmo of Fiordiligi’s firmness; Guglielmo tells a stunned Ferrando of Dorabella’s frailty, and adds a lighthearted reproach to all womankind.

Despina congratulates Dorabella; Fiordiligi rebukes her, but Dorabella replies cheerfully that Love’s commands must be obeyed. Fiordiligi decides to flee temptation: she and Dorabella will join their lovers at the front. She tells Despina to fetch two uniforms that the officers happen to have left behind. While she dons a helmet – it happens to be Ferrando’s – Ferrando himself (still in Albanian disguise) enters and woos her with an eloquence she is unable to resist. Guglielmo and Don Alfonso have been watching. Don Alfonso consoles his friends with a moral that they wryly repeat: “Così fan tutte!”

A double wedding is prepared. While three of the party drink a toast to forgetfulness of the past, the fourth, Guglielmo, declares that the glasses should hold poison. A notary appears and a marriage contract is drawn up. Suddenly, a military march is heard. Can the officers be returning? They are. The “Albanians” are bustled into an adjacent room, slip out again and return undisguised to find their beloveds more agitated than welcoming. They “discover” the notary – who, to the sisters’ surprise, turns out to be Despina – and then the marriage contract! The men go out hunting for their rivals, and return holding items of Albanian disguise to taunt Fiordiligi, Dorabella and Despina. The sisters say that Don Alfonso is to blame. He agrees; he deceived them in order to undeceive their lovers, who will henceforth be wiser. Join hands again, he says. The sisters beg for forgiveness. And all declare that people guided by reason and blessed with a sense of humor will be able to find calmness amid the tempests of life.