Act One opens in a café, where two young officers, Ferrando and Guglielmo, are bragging about the undeniable fidelity of their fiancées, the sisters Dorabella and Fiordiligi. An older friend of theirs, Don Alfonso, tells them that no women are as faithful as all that. He bets the two young men a hundred gold pieces that if they do everything he says, their lovers will betray them. Ferrando and Guglielmo eagerly accept the wager -- though their music is so stiff and ceremonial that we have to wonder just how confident they are.
Alfonso's scheme is this: The two men will pretend they've been called away to war. Then, while the women are mourning their absence, the men will return in disguise and try to seduce each other's lovers. If they succeed, Alfonso wins the bet.
In the second scene, Fiordiligi and Dorabella are heartbroken when they find their lovers are about to leave town. The sisters' servant, Despina, suggests that while the men are gone the two women should entertain themselves -- with new lovers. Their reaction is shock and indignation, but the underlying tone of the music suggests something a little different, hinting that the women might just be interested in some extracurricular hanky-panky.
When the sisters leave the room, Alfonso gets Despina to help with his plan -- for a fee, of course. He tells her that two wealthy Albanians are in town, and are eager to meet her mistresses. These "Albanians" are actually Ferrando and Guglielmo, wearing outlandish disguises. Despina agrees to introduce them to Dorabella and Fiordiligi. But when the Albanians begin to make their moves, the two women are offended, and leave in a huff. The soldiers think they’ve won the wager, but Alfonso tells them it's not over yet.
In the act's final scene, Alfonso and Despina hatch the next part of their plan. The women are in the garden, again pining for their absent lovers. The Albanians suddenly appear. They both say they can't bear any further rejection, and pretend to swallow arsenic.
Despina goes with Alfonso to find a doctor, and as the women timidly approach the supposedly dying men, Alfonso returns -- bringing Despina in disguise as a doctor. Using a dubious magnetic medical device, she revives the Albanians. Fiordiligi and Dorabella had expressed sympathy for their suicidal suitors. But when the men immediately resume their pleas for love, they're rejected all over again.
As Act Two opens, Despina tells the two women they're being silly. She thinks that any woman over the age of 15 should take advantage of this kind of situation. After all, she says, you can survive without love -- but not without lovers. The sisters act scandalized by her suggestions, but they're beginning to think Despina might be right. Dorabella persuades her sister that it could be fun to lead these guys on. She says she'll take the dark one -- Guglielmo -- leaving Fiordiligi with Ferrando. Unwittingly, the women have just agreed to switch lovers.
Despina and Alfonso bring the two couples together, and leave them alone, to let nature take its course. Both men turn on the charm -- and one of them succeeds. Guglielmo persuades Dorabella to accept a gift, a small golden heart, in return for the locket she wears -- a gift from Ferrando -- which he puts around his own neck.
When the two men get back together to compare notes, things start to go sour. Ferrando has had no luck with Fiordiligi, and he assumes Guglielmo has also failed with Dorabella. Guglielmo first hedges a bit, but when he finally reveals that Dorabella has given him her locket, Ferrando is deeply wounded. Disguising his pain as anger, Ferrando resolves to have another try with Fiordiligi.
Meanwhile, the sisters are also conferring. Dorabella has adopted Despina's carefree attitude, and is ready to marry her new lover. Yet it's Fiordiligi who feels most guilty. She rejected her Albanian -- but, deep down, wishes she had acted on her passion. As a sort of penance for those feelings, she decides that she and Dorabella must put on soldiers' uniforms and head for the battlefield to be with their true loves.
But Ferrando reappears, still as an Albanian, and Fiordiligi can't resist him any longer. She gives in while Guglielmo is watching. Now it's his turn for anguish.
Alfonso says the two men might just as well marry the women. They were bound to be unfaithful eventually, so why not make the best of it? And just then, Despina announces that the women have agreed to a double wedding.
In the final scene, the couples uneasily drink to their future happiness -- while the two men are quietly seething. The versatile Despina arrives, this time disguised as a notary, and bringing a marriage contract. The women sign it, but before the men can do the same, Alfonso suddenly announces that the sisters' former boyfriends have returned from battle. Frantically, Dorabella and Fiordiligi send the Albanians into another room.
The men sneak out the back, and return without their disguises. They feign shock and horror when they learn that their faithful lovers have signed a marriage contract. Alfonso points to the room where the Albanians are supposedly hiding, and the men go in with swords drawn. When they return carrying the disguises, the women finally realize they've been tricked.
Alfonso tells the four lovers that it's all for the best -- everyone is wiser now. They've seen each other as they really are, and can all have a good laugh at their own expense. There are apologies all around, and everything is forgiven. But even as the finale rings out, no one seems ready to laugh, and we're left wondering if the couples really have the same worry-free future they looked forward to as the opera began.