The story is set in Naples and tells the tale of two young men, Ferrando and Guglielmo, who are in love with two sisters, Fiordiligi and Dorabella. The men are convinced that their beloveds are faithful, but Don Alfonso, an older friend, challenges them to a wager. He bets that he can make the women fall in love with other men.
Don Alfonso's plan involves convincing Ferrando and Guglielmo to pretend to go off to war. As soon as they leave, he introduces two new men to the women, Donati and Iocane. The men woo the women, and at first, they resist their advances. However, as time passes, they begin to weaken.
Eventually, Ferrando and Guglielmo return from the war. They are shocked and heartbroken to find that their fiancées have been unfaithful. However, Don Alfonso reveals that the whole thing was a trick. He wanted to show the men that women are not always as faithful as they seem.
Cosi fan tutte is a comic opera, but it also contains elements of tragedy. The characters are faced with difficult choices, and they must ultimately decide what is most important to them. The opera is a exploration of love, fidelity, and the nature of human relationships.