In a Naples café two young officers, Guglielmo and Ferrando, are deep in conversation with the elderly philosopher Don Alfonso. Both are convinced that their mistresses – named Fiordiligi and Dorabella respectively – are utterly faithful, but the cynical old man warns them not to harbour dangerous illusions. Irritated, the young men propose fighting a duel to defend the honour of their lovers. But the philosopher makes a bet with them: if he can demonstrate within twenty-four hours that their mistresses are disloyal he will win a hundred pieces of gold. Should lose, Alfonso will have to pay this sum. The officers accept the wager, agreeing to follow Alfonso’s every instruction.
Fiordiligi and Dorabella are in their garden, looking adoringly at the portraits of their lovers. Don Alfonso brings them sad tidings: the king has recalled the two officers to the battlefield. The girls bid their departing lovers a heart-rending and tearful farewell, but Alfonso roars with laughter at the exchange of tender promises and prolonged farewells. As the boat draws away Fiordiligi and Dorabella, now joined by the philosopher, wish their lovers a smooth journey. Despina, the girls’ maid, has been informed of the sad event. She encourages her mistresses not to overdramatize the situation and to console themselves by finding new lovers. The girls flounce off angrily. Alfonso enlists Despina’s help, bribing her with a gold coin to smuggle two «distinguished gentlemen» into the household. Despina is dumbfounded by the appearance of the two suitors – a pair of extravagantly dressed and heavily mustachioed Albanian noblemen whom the servant girl fails to recognize as being none other than Ferrando and Guglielmo. Fiordiligi and Dorabella reproach Despina for having let them in, but Alfonso comes forward, pretending that the Albanians are old friends of his. Fiordiligi reiterates their unshakeable loyalty, demanding that it be respected. Guglielmo clumsily seeks out their favours, but the offended girls disdainfully take their leave. Their behavior raises the spirits of the two young men, who act as though they had already won the bet, despite Alfonso’s warnings that for the time being they must continue to obey his orders.
Despina tells the philosopher that she has a plan that will guarantee her mistresses surrender. Soon afterwards the ‘Albanians’ join the girls in the garden. Feigning total despair they both swallow the contents of two phials of (alleged) arsenic. Alfonso begs the sisters to take pity on the two creatures and, as he hurries away for the doctor, Fiordiligi and Dorabella begin to soften at this pathetic gesture of devotion. A rather curious doctor (Despina in disguise) arrives on the scene and, brandishing a special magnet, restores the «dying» men to life. The Albanians are now considered to be «interesting» by the two girls, who prop them up and watch over them attentively. Ferrando presses his affections on Fiordiligi, whilst Guglielmo turns to Dorabella (thus swapping their real-life mistresses). The fact that the women respond to their untimely demands for kisses with an angry outburst is a clear sign that they are on the point of surrender. In the meantime the two ‘Albanians’ begin to ask themselves whether the girls’ rage is genuine and fear the worst.
Despina urges her mistresses not to lose their chances. She suggests that, in order to save their reputations, they can pretend that she, Despina, is the object of the foreigners’ attentions. Dorabella is of the opinion that the suitors should be admitted, simply so that she and her sister may «have a little fun and not die of melancholy». Fiordiligi is persuaded and tells Dorabella to choose the man she prefers. Dorabella selects Guglielmo, leaving Ferrando to Fiordiligi, who is apparently quite happy with this state of affairs: the exchange of couples is now complete!
Alfonso tells the women that the Albanians have organized a serenade in their honour in the garden. The two suitors, accompanied by a band of musicians, entreat «friendly breezes» to waft their sighs across to the scornful beauties. Dorabella is the first to succumb to their ardent pleas, accepting Guglielmo’s gift of heart-shaped locket to replace a medallion containing Ferrando’s portrait. Fiordiligi is in a state of turmoil but, still refusing to yield, she finds the courage to send Ferrando away. Once alone, however, she admits her fonder feelings for him. The two ‘Albanians’ take stock of the situation. Guglielmo is relieved to learn that Fiordiligi has not yielded to his friend’s advances but, with illconcealed satisfaction, he is bound to confess that Dorabella has not been quite so virtuous. In despair, Ferrando plots his revenge whilst Guglielmo, certain that he has won the bet, asks Alfonso for his fifty pieces of gold. The wise old philosopher reminds him, however, that the twenty-four hours are not yet up.
Fiordiligi is anxiously pacing up and down in her chambers. Ignoring the protests of her sister – who cannot wait to marry her new suitor – she decides that the only way to save her honour is to join Guglielmo at the army camp, disguised as an officer. As Fiordiligi prepares to take her leave, Ferrando enters. Driven by desire for vengeance the young man persistently plays his last trump card, pretending to seek solace in death. Fiordiligi can no longer resist him and falls into his arms. Guglielmo and Alfonso have been watching the encounter from a hiding place and the former now lets forth a stream of furious against his treacherous mistress. Together with Ferrando he plans how best to punish the faithless women and Alfonso has a suggestion to make: «Marry them». The two young men balk at the idea but the old philosopher explains that since nature provides no exceptions to the rule, they may just as well hold on to these women – after all, «così fan tutte»: they’re all alike! Despina brings the good news that the «dear ladies» have accepted the ‘Albanians’ marriage proposals and busies herself preparing a new disguise.
A magnificent wedding party begin. Whilst the bridegrooms exchange congratulations, the chorus ironically expresses its hopes that the «amiable brides» will be «like hens prolific, with children that equal them in beauty». Alfonso ushers in a notary (once again Despina in disguise). The marriage contract is drawn up and just as the women have signed it, a chorus of male voices extolling the soldier’s life can be heard in the distance. The two officers are returning to their loved ones! The ‘Albanians’ pretend to hide with the notary in the adjoining room. Shortly afterwards Guglielmo and Ferrando enter and passionately embrace their «adorable brides», who greet them, trembling and strangely silent. Using his trunk as an excuse to go into the next room, Guglielmo discovers the ‘lawyer’. But the crafty Despina takes off her cloak and disguise, saying that she had dressed up to go to a masked ball. Following Alfonso’s suggestion, the officers retrieve the marriage contract from the floor and fly into a rage when they see that it bears the signatures of their mistresses. The girls have no alternative but to admit the truth, pointing accusing fingers at Alfonso and Despina as real culprits. The two officers go into the next room and emerge carrying the ‘Albanians’ costumes, returning Ferrando’s portrait to Dorabella and the doctor’s magnet to Despina. All is now clear. The last word goes to the philosopher who reunites the couples, cheering up the two young friends with his logical cynicism: since their illusions have been shattered, they might as well get married. Everyone agrees to his proposal with relief: «Lucky is the man who / sees a good side to everything / and who, in the face of all incidents and events, / allows himself to be guided by reason».