Scene 1: A coffeehouse
In a cafe, Ferrando and Guglielmo (two officers) express certainty 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, like all women, are fickle. The wager is accepted: the two officers will pretend to have been called off to war; soon thereafter they will return in disguise and each attempt to seduce the other's lover. The scene shifts to the two women, 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 (quintet:
Sento, o Dio, che questo piedo è restio—"I feel, oh God, that my foot is reluctant"). As the boat with the men sails off to sea, Alfonso and the sisters wish them safe travel (trio:
Soave sia il vento—"May the wind be gentle"), then Alfonso, left alone, gloatingly predicts that the women (like all women) will prove unfaithful. (arioso:
Oh, poverini, per femmina giocar cento zecchini?—"Oh, poor little ones, to wager 100 sequins on a woman").
Scene 2: A room in the sisters' home
Despina, the maid, arrives and asks what is wrong. Dorabella bemoans the torment of having been left alone (aria:
Smanie implacabili—"Torments implacable"). Despina mocks the sisters, advising them to take new lovers while their betrotheds are away (aria:
In uomini, in soldati, sperare fedeltà?—"In men, in soldiers, you hope for faithfulness?"). After they leave, Alfonso arrives. He fears Despina will recognize the men through their disguises, so he bribes her into helping him to 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 (aria:
Non siate ritrosi—"Don't be shy"), but to no avail. Fiordiligi pledges to remain faithful. (aria:
Come scoglio—"Like a rock"). Ferrando, left alone and sensing victory, praises his love (aria:
Un'aura amorosa—"A loving breath").
Scene 3: A garden
The sisters are still pining. 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 (aria: Una donna a quindici anni—"A fifteen year old woman"). 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 (duet:
Prenderò quel brunettino—"I will take the dark one").
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 (duet:
Il core vi dono—"I give you my heart"). Ferrando is less successful with Fiordiligi (Ferrando's aria:
Ah, lo veggio—"Ah, I see it," and Fiordiligi's aria: Per pietà, ben mio, perdona—"Please, my beloved, forgive"), 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 (aria:
Donne mie, la fate a tanti—"My ladies, you do it to so many") but then gloats, because his betrothed is faithful.
Scene 3: The sisters' room
Dorabella admits her indiscretion to Fiordiligi (È amore un ladroncello—"Love is a little thief"). 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 (duet:
Fra gli amplessi—"In the embraces"). 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."
The scene 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.