Best buddies Ferrando and Guglielmo are boasting about their respective girlfriends, the sisters Dorabella and Fiordiligi. Their older friend, the cynical casino manager Don Alfonso, scorns their naivete, claiming that there’s no such thing as a faithful woman and that he can prove it in one day’s time. Don Alfonso seizes a timely opportunity and the three friends agree to a wager based on an elaborate prank, where Ferrando and Guglielmo will pretend that they’ve been called abroad to look after their investments, as a result of the Wall Street Crash. After a sham farewell, they will return in disguise as lifeguards and woo each other’s girlfriend. The bettors are confident and Don Alfonso is covertly gleeful as he breaks the sad news of the boys’ departure to Dorabella and Fiordiligi, who have been extolling their boyfriends’ merits. The mood darkens as they watch their lovers depart. Will the ruse be more than the boys ever bargained for?

With their lovers gone, the sisters feel lonely and despondent. Head of Housekeeping, Despina, who happens to be Don Alfonso’s wily sidekick, offers a remedy: simply take new lovers until the boys are back, as soldiers do while they are away. Outraged, they reject her advice. Meanwhile, the boyfriends return dressed as lifeguards. They swear passionate love to the girls, but get nowhere with this approach. Their mates’ intransigence heartens them.

With the sisters moping and pining, Despina agrees to help Don Alfonso with the ruse. Suddenly Ferrando and Guglielmo burst in with even more exaggerated romantic gestures and threaten suicide if Fiordiligi and Dorabella continue to refuse their love.   The boys pretend to drink poison and Despina, disguised as a physician, administers phony medical treatment. Though moved by their desperate actions, the sisters refuse even to kiss the disguised pranksters. But is their resolve weakening?

In the wake of their previous medical misadventure, the four lovers converse awkwardly with their switched partners. When Ferrando leaves with Fiordiligi, Guglielmo grows more amorous with Dorabella, whose resistance weakens. Before long, they exchange tokens of love. When Ferrando learns that Dorabella’s faithfulness is compromised, he is enraged. Guglielmo’s sympathy gives way to gloating.

Dorabella confesses her lapse to her sister, who is alarmed. Wondering how she can hold out, Fiordiligi decides to go in search of Guglielmo abroad. Her resolve seems firm, but before she can leave, Ferrando arrives and pursues her more urgently. She finally relents. The triumphant Ferrando turns earlier gloating back on the now-distraught Guglielmo as the jaded Alfonso, having won his bet, crows that “Così fan tutte”—”Women are like that.”

A sham double-wedding is staged for the sisters’ final entrapment; Despina, disguised as a notary, officiates. Soon, Alfonso confirms the sisters’ worst fears: Ferrando and Guglielmo are returning from their travels. When they arrive and profess their steadfast love, Alfonso unveils the faked marriage contract, revealing the prank to the hapless Fiordiligi and Dorabella. In a final ensemble, the opera’s principals agree that they’ve gained wisdom from their experience, but the moral lesson is up to the observer to decide.