Offended by the misogynist remarks of their old friend, the philosopher Don Alfonso, Ferrando and Guglielmo agree to lay wagers on the outcome of a game he has designed to test the fidelity of their respective sweethearts, Dorabella and Fiordiligi. The two young men agree to obey the instructions of Alfonso (who has enlisted the help of Despina, the women's assistant): they feign a heartbroken departure to the front, only to return a few hours later disguised as dashing Albanians, eager to win the favours of the abandoned sisters. The young women put up an outraged defence of their virtue but before long they are yielding to the exotic advances of the strangers. The conspirators gradually become enmeshed by their own plot and are dismayed to find they are as susceptible as their victims. Events culminate in a double wedding and all four lovers discover that no-one is exempt from the secret inconsistencies of love.
Two officers, Ferrando who is in love with Dorabella, and Guglielmo who is in love with her sister Fiordiligi, are discussing women with their old friend Don Alfonso. Alfonso maintains that faithfulness in women is like the phoenix, often talked about but never seen (È la fede delle femmine). Ferrando and Guglielmo protest that their two sweethearts are exceptions.
Alfonso bets the men 100 gold pieces that the women's fidelity would not survive a day of their lovers' absence; he will prove it if the men promise to obey him and, in disguise, woo each others' betrothed. The men agree and swear not to give even a hint of their bet to their 'Penelopes'. Ferrando plans to spend his winnings on a serenade to his sweetheart (Una bella serenata), Guglielmo on a banquet for his.
Fiordiligi and Dorabella rapturously sing the praises of their lovers (Ah guarda sorella). They are interrupted by Alfonso, bringing them bad news (Vorrei dir): Ferrando and Guglielmo have been called up for active service. They come to bid the women farewell (Sento, o Dio) and console them with the idea that fate will return them safely (Al fato dan legge). A march is heard, and a song in praise of the military life (Bella vita militar). Fiordiligi and Dorabella beg the men to write to them every day (Di scrivermi ogni giorno). After they have left, Alfonso and the sisters offer a prayer for the soldiers' safety (Soave sia il vento). Alone, Alfonso congratulates himself on the success of his plot so far.
Despina has prepared chocolate for Fiordiligi and Dorabella and is sampling it when they appear. The women are beside themselves with grief at the men's absence and Dorabella expresses her despair (Smanie implacabili). Despina brushes their sorrow aside: men, especially soldiers, are unfaithful, and the women should find other lovers (In uomini, in soldati).
Alfonso appears and bribes Despina to help him introduce two Albanian visitors (Ferrando and Guglielmo in disguise) to the sisters (Alla bella Despinetta). The women are outraged at this intrusion, even though Alfonso claims the men as his friends, and they resist the newcomers' advances. The Albanians beg forgiveness for their importunity. Fiordiligi declares that she stands firm as a rock (Come scoglio). Guglielmo begs Fiordiligi and Dorabella not to be shy (Non siate ritrosi) but the sisters leave in indignation. Guglielmo and Ferrando are delighted that it seems they have as good as won their bet, but Guglielmo realizes he is rather enjoying the playful atmosphere of love (Un'aura amorosa).
Fiordiligi and Dorabella believe their fate has changed (Ah, che tutta in un momento). The Albanians, apparently distraught at the women's coldness, rush in and drink poison. Alfonso and Despina urge Fiordiligi and Dorabella to show compassion to the men while they go and get help. Despina reappears, disguised as a doctor, and revives the Albanians by magnetizing the poison from them, according to Mesmer's latest principles. Impressed by the men's devotion, the sisters comfort them - but decline to kiss them.
Despina tries to persuade Fiordiligi and Dorabella to allow the foreigners to call again: young women who know how to attract men can have them at their mercy (Una donna a quindici anni). The women have already decided there is no harm in a mild flirtation; Dorabella has chosen the dark-haired suitor (Prenderò quel brunettino) and Fiordiligi prefers the blond - conveniently opting for each other's lover.
The Albanians serenade the two women (Secondate, aurette amichi), after which the four are tongue-tied with embarrassment. Alfonso and Despina save the day, encouraging the couples to join hands and go for a stroll (La mano a me date).
Each pair is now alone for the first time. Guglielmo soon persuades Dorabella to succumb to his charms (Il core vi dono) and replaces Ferrando's portrait, which she wears on a necklace, with a heart of his own. Fiordiligi, however, resists Ferrando's protestations of devotion but, once he has left, she reveals her inner turmoil, imploring the absent Guglielmo for forgiveness (Per pietà).
Ferrando and Guglielmo compare notes. Ferrando is furious to learn of Dorabella's acquiescence, and Guglielmo now feels he can echo Alfonso's views about women (Donne mie la fate a tanti). Ferrando denounces Dorabella's faithlessness but admits he still loves her (Tradito, schernito). Alfonso takes advantage of Ferrando's despair to urge him into another attempt to seduce Fiordiligi.
Despina praises Dorabella's good sense in having succumbed to her new lover. Dorabella, happy that she has done so, tries to persuade Fiordiligi to follow suit (È amore un ladroncello): love is a thief, but brings pleasure. Fiordiligi resolves to save the situation; the only solution is for her to disguise herself in one of Guglielmo's uniforms and go and find him on the battlefield. But as she prepares to go, Ferrando enters, and she finally yields to his entreaties to stay (Fra gli amplessi). Guglielmo witnesses this and realizes that Alfonso has won his bet.
Alfonso tells the men their only revenge is to marry their new conquests; all men accuse women of inconstancy (Tutti accusan le donne) but women should be forgiven because they all behave like that: 'Così fan tutte'. Despina goes to find a notary for the forthcoming wedding.
Despina wants everyone to hurry with the marriage preparations (Fate presto, o cari amici). The couples thank her for bringing them together and drink a toast to oblivion of the past. Alfonso announces the arrival of a 'notary' (Despina again), who goes through the marriage contracts, which the women sign.
Suddenly they hear a march: their old lovers must be returning. The Albanians disappear and Ferrando and Guglielmo arrive, straight from the battle. They notice that the women are reserved in their welcome, and feign puzzlement at their reception. The men discover the notary, now revealed as Despina, and the marriage contract. The sisters admit their guilt and Alfonso takes the men off to find the Albanians; Ferrando and Guglielmo return, wearing part of their Albanian disguise. The deception is revealed. Ferrando greets Fiordiligi, Guglielmo returns the medallion to Dorabella and both men address Despina as the doctor. Alfonso admits that the plot was his idea and urges the four to laugh it off. The women express their remorse and the men accept pleas for forgiveness. The experiment is over and all may learn a lesson from it.