Act I
Don Alfonso, an old philosopher, tells his young friends Ferrando and Guglielmo that women are unfaithful and cannot be trusted. Outraged, Ferrando and Guglielmo brag about their fiancées’ constancy. Don Alfonso offers to put their fidelity to the test in a wager if the young men agree not to reveal anything to their fiancées, the sisters Dorabella and Fiordiligi, Don Alfonso tells the heartbroken women that their fiancés are leaving.
According to Alfonso's plan, the two men will pretend that they have been called away to war. While the women are mourning their absence, the men will return in disguise and try their best to seduce each other's fiancées. If they succeed, Alfonso wins the bet.
The sister’s maid Despina suggests that while the men are gone the sisters could enjoy themselves — with other men. Alfonso bribes Despina to help with his scheme. He tells her that two ‘Albanians’ have arrived, eager to meet her mistresses. Despina agrees to introduce them to Dorabella and Fiordiligi. When the fake Albanians make their moves, the two offended women remain firm. The two friends rejoice to see Don Alfonso’s plan fail, but Alfonso advises patience…
Act II
Don Alfonso’s ruse is indeed beginning to work. The sisters act scandalized by Despina’s suggestion of harmless flirtation, although they are tempted. Dorabella convinces Fiordiligi that it might be entertaining to take the men for a ride, each unwittingly choosing the other's fiancé. Guglielmo quickly wins over Dorabella.
Though she resists longer than her sister, Fiordiligi ends up giving in to Ferrando. Alfonso suggests the two men might just as well marry the sisters. Just as Despina, dressed as a notary, is drawing up a counterfeit marriage certificate for the newly formed couples, a trumpet announces their real fiancés’ return.
The men reappear without their disguises and pretend shock at what is afoot, before exposing their plot. The women finally realize that they have been deceived. Alfonso tells everyone that it’s for the best: they have shown their true selves and should all laugh at their own expense. The couples forgive each other and are reunited. Nonetheless, the atmosphere is uneasy. Can the mistrust and betrayal on both sides ever really be forgotten?