Act I

In a tavern
Don Alfonso and two young officers, Ferrando and Guglielmo, are discussing women. The soldiers insist that their sweethearts are beyond temptation and would never sway from virtuous devotion. Alfonso remarks that women are not so angelic and raises the ire of the soldiers. In an effort to educate his naive friends Alfonso suggests a little wager: if the men will do as he directs, in only twenty-four hours he will have proved his point. With great confidence that their girlfriends would never consider anyone else, the young men accept his challenge.

In their garden
The sisters Dorabella and Fiordiligi revel in their love for Guglielmo and Ferrando. Alfonso arrives with the sad news that their lovers have been called with their regiment to the front. The men appear and all engage in elaborate farewells. Alone, Alfonso delivers one last jeer at women's inconstancy.

In their sitting room
The maid Despina offers the women their morning chocolate and advises the inconsolable girls to forget their sorrow with the aid of new, temporary lovers. Her mistresses are affronted by Despina's capricious philosophy and leave the room.

When the sister have gone, Alfonso comes to seek Despina's assistance in his deception. With the aid of a bribe, he convinces her to introduce two foreign friends to Dorabella and Fiordiligi. When the foreigners arrive, the ladies are scandalized at the intrusion. The men, disguised as Albanians, enthusiastically declare great admiration for the sisters, who are utterly repulsed. They do not recognize the disguised men as their lovers. In a tirade, Fiordiligi likens her fidelity to an immovable rock. The men are thrilled at the failure of Alfonso's plot, but he warns them that the bet isn't won yet. Left alone, Ferrando blissfully reiterates his passion for Dorabella.

In their garden
The sisters unite in despair. Suddenly, the Albanians stagger in, pretending to have taken poison. Alfonso and Despina run for a doctor while the ladies rush to help the men. Despina returns disguised as a doctor and uses Mesmer's vogue invention, an enormous magnet, to cure the ailing Albanians. During the emergency the women are greatly moved, but they angrily refuse to grant even a kiss once the men are revived.

Act II

In their boudoir
Despina urges Fiordiligi and Dorabella to relent and give in to their suitors. Dorabella agrees, and the reluctant Fiordiligi gives way.

In the garden
The men have arranged a serenade. Guglielmo pairs off with Dorabella while Ferrando continues to woo Fiordiligi, though she seems to have no interest in pursuing the courtship. However, Fiordiligi admits to herself that he has touched her heart, and she hopes her absent lover will forgive these faithless thoughts. When the men compare notes, Guglielmo is glad to see that Fiordiligi stands fast. Ferrando, however, is dismayed at Dorabella's yielding. His angers amuses Guglielmo, who comments on the waywardness of the fairer sex.

In their boudoir
The sisters confess to Despina that they have lost their hearts. Alone, the troubled Fiordiligi refuses to act on her feelings and decides to assume a disguise of her own and, as a soldier, join her lover at the battle. When, in response to her continued refusals, Ferrando threatens suicide, Fiordiligi is overcome and capitulates. When Guglelmo hears of this, he is furious but Alfonso counsels forgiveness: "Così fan tutte!" (That's the way all women behave!), he claims.

In a receiving room
A private wedding party is arranged for the sisters and their Albanians. The servants salute the apparently happy couples and Alfonso brings in a notary, who is actually Despina in yet another disguise. As the ladies sign the marriage contract, familiar martial strains from outside draw Alfonso to a window: The former lovers are returning with their regiment! In a panic, Fiordiligi and Dorabella push their intended husbands from the room. The men reappear in regimental uniforms, to enact a seeming reunion, but they instantly storm the ladies when the wedding contracts are "discovered." Alfonso then reveals the Albanian disguises and hails the triumph of reason over unrealistic expectations, and forgiveness prevails.