Time: 18th century.
In the morning, Ferrando and Guglielmo joyously proclaim the eternal faithfulness of their beloved sweethearts Dorabella and Fiordiligi. Don Alfonso overhears and wagers that if given a day, and they do everything he tells them, he can prove their sweethearts' affections are fickle (as all women are.).
While the two sisters are comparing their men, Don Alfonso enters and brings news that the two officers have been ordered away. Ferrando and Guglielmo bid a reluctant farewell (quintet: Sento, o Dio, che questo piedo è restio—"I feel, oh God, that my foot is reluctant"). As the boat sets sail the grief-stricken sisters and Don Alfonso wish them a gentle voyage (trio: Soave sia il vento—"May the wind be gentle".) Alfonso alone, gloats on his certain winnings (arioso: Oh, poverini, per femmina giocar cento zecchini?—"Oh, poor little ones, to wager 100 sequins on a woman").
Dorabella bemoans the torment of having been left alone (aria: Smanie implacabili—"Torments implacable"). Despina consoles then mock the sisters, advising them to consider new lovers over old lovers (aria: In uomini, in soldati, sperare fedeltá?—"In men, in soldiers, you hope for faithfulness?"). The sisters' exit scandalized at this suggestion. Don Alfonso bribes Despina to help introduce new suitors to the sisters. Ferrando and Guglielmo return disguised as Albanians and attempt to seduce each others' fiancee. The sisters are alarmed at the presence of strange men in their homes. Guglielmo tries to woo by pointing out his manly attributes (aria: Non siate ritrosi—"Don't be shy"), but to no avail (aria: Come Scoglio—"Like a rock"). Ferrando, alone, amorously praises his love (aria: Un aura amorosa—"A loving breath").
As the sisters' pine in the garden, the disguised men burst in and threaten to poison themselves if they are not allowed the chance to woo their loves. Alfonso attempts to intervene but the men, unrequited, drink the poison and pass out. Despina is called she urges the sisters to attend to the men while she gets a doctor. Despina returns disguised as a doctor, who, through the use of a large magnet, revives the love-stricken men. The revived men demand a kiss of the goddesses who stand before them. Fiordiligi and Dorabella waver as Despina and Alfonso watch.
In the afternoon, Despina urges the sisters to choose between the men and each picks the other's fiance (aria: Una donna a quindici anni—"A fifteen-year-old woman"). Dorabella confesses that she is tempted but Fiordiligi remains steadfast (duet: Prenderó quel brunettino"—"You take the darker one").
In the garden, Dorabella and the disguised Guglielmo pair off, as do the other two. After an awkward serenade, Ferrando departs with Fiordiligi. Guglielmo succeeds in wooing Dorabella and she surrenders Ferrando's portrait in exchange for a heart-shaped charm (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 pietá, ben mio, perdona—"Please, my beloved, forgive"), and he is enraged to discover Guglielmo has been so quickly successful. Guglielmo at first sympathizes with Ferrando (aria: Donne mie, la fate a tanti—"My ladies, you do it to so many") but then gloats, as his fiance is faithful.
In the sister's room Dorabella admits her indiscretion to Fiordiligi (É amore un ladroncello—"Is love a little thief"). Fiordiligi, upset, decides they must leave and join their departed lovers. Before she can leave, Ferrando arrives and continues his seduction. Fiordiligi finally gives in (duet: Tra gli amplessi—"In the embraces"). Guglielmo is distraught and after a little gloating from Ferrando, they both wish to call off the wager. Alfonso tells the men to forgive their fiancees and continue the wager. After all: Cosi fan tutte—"All women are like that." The title phrase is sung by the three male singers.
The final scene begins as a double wedding for the sisters and their disguised grooms. Despina, in disguise as a notary, presents the marriage contract, which all sign. Directly thereafter, military music is heard indicating the return of the officers. The sisters urge the notary and their newlyweds to hide while they confront their past lovers. Don Alfonso let slip the marriage and the two men discover the notary hiding. The crest-fallen sisters admit their indiscretion and beg forgiveness. The men reveal their deception, Despina is revealed as the notary, and the sisters realize they have been duped. All is forgiven, as the entire group praises the ability to accept the fickleness of life.