Don Alfonso and his young military friends, Ferrando and Guglielmo, are discussing the fidelity of women. Don Alfonso claims that all women are fickle. Ferrando and Guglielmo object to this assertion, on the contrary they believe that two sisters – Fiodilligi and Dorabella whom they are in love with, will never be unfaithful.. Don Alfonso thereupon wagers that by the end of the day he will have proved his point. He proposes that Ferrando and Guglielmo pretend that they have received orders to leave Naples. In reality they will return in disguise, each to pay court to his friend’s sweetheart.
Fiordiligi and Dorabella are singing the praises of their lovers, when Don Alfonso enters and tells them of the new military order. The young will have to depart for battlefield. When the young man arrive and bid the two girls affectionate farewell. Despina, the maid, is preparing breakfast when the sisters enter and tell her of their loss. Calmly she advises them to amuse themselves while their lovers are away, an advice that meets with horrified rejection. Don Alfonso, who has overheard this conversation, sees in Despina a possible ally for his plot and persuades her to connive with him. He lets Guglielmo and Ferrando into the house, disguised as Albanians. The sisters are shocked to find two strangers in their home, and even more shocked when the two begin courting them. Don Alfonso pretends they are old friends of his and pleads their cause. But Fiordiligi staunchly proclaims her fidelity to her absent lover.
The sisters are lamenting the departure of their lovers, when the two Albanians suddenly rush in. They melodramatically swallow what appears to be poison and collapse. Despina and Don Alfonso go in search of a doctor, leaving the sisters to watch over the apparently dying men. The doctor, Despina in disguise, announces that he is a follower of the famous Dr. Mesmer and pretends to extract the poison from the two suicides, who, as they recover, renew their ardent protestations of love.
After listening to Despina, the sisters decide there would be no harm after all in a little innocent flirtation. Dorabella, Ferrando’s betrothed, chooses Guglielmo while Fiordiligi, Guglielmo’s betrothed, chooses Ferrando.
The Albanian visitors have arranged a serenade for the two sisters, and Fiordiligi goes off for a stroll with Ferrando and the disguised Guglielmo courts Dorabella. To his astonishment, she yields and exchanges a locket with him. They go out together as Ferrando returns with Fiordiligi, who has remained outwardly steadfast. The two young men then meet to exchange news of their progress. Ferrando is overcome with grief when Guglielmo shows him the faithless Dorabella’s locket.
While Don Alfonso and Guglielmo conceal themselves, Ferrando makes another attack upon Fiordiligi’s tenacity. Much to Guglielmo’s distress, she finally yields. Don Alfonso has won the bet and he tries to console the two chastened young men with the moral that women always act in this manner -- cosi fan tutte.
Celebration of the sisters’ betrothal to the Albanians is in full swing, when a drumroll is heard in the distance, announcing the supposed return of the officers. In simulated panic the Albanians and the notary (again Despina in disguise) hide. A moment later Ferrando and Guglielmo appear in uniform. They seem to be puzzled by the cool reception they receive until they discover the marriage contract and the notary. With drawn swords they rush out in pursuit of their rivals, only to return immediately with their disguises in their hands. The ruse has been revealed and nothing remains but for the lovers to follow Don Alfonso’s advice to forgive and forget.