Ferrando and Guglielmo (two officers) claim that their fiancées (Dorabella and Fiordiligi, respectively) will be eternally faithful. Don Alfonso joins the discussion and lays a wager with the two officers, claiming he can prove in a day's time that these two women (like all women) are fickle. The wager is accepted: the two officers will pretend to have been called off to war; soon thereafter they shall return in disguise and attempt to seduce each other's lover. The scene shifts to the two women (they are sisters) who are praising their men. Alfonso arrives to announce the bad news: the officers have been called off to war. Ferrando and Guglielmo arrive, brokenhearted, and bid farewell. As the boat with the men sails off to sea, Alfonso and the sisters wish them safe travel, then Alfonso, left alone, rails against the fickleness of women
Scene 2: A room in the sisters' home.
Despina, their maid, arrives and asks what is wrong. Dorabella bemoans the torment of having been left alone. Despina mocks the sisters, advising them to consider new lovers over old lovers. After they depart, Alfonso arrives upon the scene. He fears Despina will recognize the men through their disguises, so he bribes her into helping him win the bet. The two men then arrive, dressed as mustachioed Albanians. The sisters enter and are alarmed by the presence of strange men in their home. The "Albanians" attempt to win over the sisters, Guglielmo going so far as to point out all of his manly attributes, but to no avail. Ferrando, left alone and sensing victory, praises his love.
Scene 3: A garden
The sisters are still pining. But Despina has asked Don Alfonso to let her take over the seduction plan—and suddenly, the "Albanians" burst in the scene and threaten to poison themselves if they are not allowed the chance to woo the sisters. As Alfonso tries to calm them, they drink the poison and pass out. Soon thereafter, a doctor arrives on the scene (Despina in disguise), who, through use of a large magnet (see animal magnetism), is able to revive the "Albanians". The revived men, hallucinating, demand a kiss of the goddesses who stand before them. The sisters refuse, even as Alfonso and the doctor (Despina) urge them to acquiesce.
Scene 1: The sisters' bedroom
Despina urges them to succumb to the "Albanians"' overtures. After she leaves, Dorabella confesses to Fiordiligi that she is tempted, and the two agree that a mere flirtation will do no harm and will help them pass the time while they wait for their lovers to return.
Scene 2: The garden
Dorabella and the disguised Guglielmo pair off, as do the other two. The conversation is haltingly uncomfortable, and Ferrando departs with Fiordiligi. Now alone, Guglielmo attempts to woo Dorabella. She does not resist strongly, and soon she has given him a medallion (with Ferrando's portrait inside) in exchange for a heart-shaped locket. Ferrando is less successful with Fiordiligi, so he is enraged when he later finds out from Guglielmo that the medallion with his portrait has been so quickly given away to a new lover. Guglielmo at first sympathises with Ferrando but then gloats, because his betrothed is faithful.
Scene 3: The sister's room
Dorabella admits her indiscretion to Fiordiligi. Fiordiligi, upset by this development, decides to go to the army and find her betrothed. Before she can leave, though, Ferrando arrives and continues his attempted seduction. Fiordiligi finally succumbs and falls into his arms. Guglielmo is distraught while Ferrando turns Guglielmo's earlier gloating back on him. Alfonso, winner of the wager, tells the men to forgive their fiancées. After all: Così fan tutte—"All women are like that."
It begins as a double wedding for the sisters and their "Albanian" grooms. Despina, in disguise as a notary, presents the marriage contract, which all sign. Directly thereafter, military music is heard in the distance, indicating the return of the officers. Alfonso confirms the sisters' fears: Ferrando and Guglielmo are on their way to the house. The "Albanians" hurry off to hide (actually, to change out of their disguises). They return as the officers, professing their love. Alfonso drops the marriage contract in front of the officers, and, when they read it, they become enraged. They then depart and return moments later, half in Albanian disguise, half as officers. Despina has been revealed to be the notary, and the sisters realize they have been duped. All is ultimately forgiven, as the entire group praises the ability to accept life's unavoidable good times and bad times.