Two engaged to be married couples’ lives are thrown out of kilter in a day.
Guglielmo and Ferrando are sure of their fiancées’, the sisters Fiordiligi and Dorabella, fidelity. Don Alfonso, acquainted with both men, dubious about female constancy, wagers: he can prove the girls can’t stay faithful for twenty-four hours. Provided both men adhere strictly to Alfonso’s rules. Guglielmo and Ferrando are convinced they’ll win the bet.
Fiordiligi and Dorabella are waiting for their lovers. Don Alfonso, livid, bursts in saying Guglielmo and Ferrando have just been conscripted to fight in a war. The men take their leave from their fiancées. The sisters are distraught. Despina, their maid, pokes fun at their despair and advises them to find new lovers. Don Alfonso, in exchange for money, makes Despina his accomplice, letting her in – to a degree – on his plan: She helps introduce the disguised men to her mistresses, without their being recognised. The sisters are outraged and want to get rid of the pushy »strangers« at once. But Don Alfonso greets them as old friends, who should be welcomed. Guglielmo and Ferrando’s first attempts to seduce the women fail. Both men are triumphant, but the bet’s deadline is hours away. Further attempts are made: Guglielmo and Ferrando pretend, heartbroken, they’ve taken poison, making the sisters feel sorry for them. Despina, disguised as a doctor, administers to the apparently dead men. They slowly come to, demanding kisses to help them recover. Fiordiligi and Dorabella are horrified. They refuse, and flee.
Act 2 Despina gives her mistresses advice on how to handle men. They allow themselves be talked into a rendezvous with the »strangers« and take their pick. Swapping, without realising it, their original partners. Don Alfonso and Despina manoeuvre the re-shuffled couples. Guglielmo manages to win Dorabella’s heart. Fiordiligi’s steadfast, but wavering. Ferrando’s gutted when he hears about Guglielmo’s success, who grumbles about all women being unfaithful, with he and his bride being the exception to all rules. Fiordiligi sees only one way to stop falling in love. By joining her fiancé at war dressed as a soldier. But Ferrando pleads with her again and she gives in. Now Guglielmo’s devastated too, Ferrando mocks him. They decide to punish their fiancées. Don Alfonso consoles them: All women are like that, and if Ferrando and Guglielmo love them, they should marry them. Despina and Don Alfonso organise a sham double-wedding. Fiordiligi and Dorabella have decided to say »I do« to their new paramours. Despina, disguised as a notary, gets the sisters to sign marriage contracts. Don Alfonso concludes the game: the unexpected return of their original fiancés is announced. The bogus notary’s found along with the signed marriage contracts. Guglielmo and Ferrando remove their disguises, revealing the sisters’ newly-wedded husbands’ true identities. Guglielmo and Ferrando furiously accuse the sisters of the infidelity, they feel guilty of causing. Don Alfonso announces that he staged it all to teach all four a lesson. The couples are reconciled and realise they’ve learnt a lot for life from his experiment.
All six draw the conclusion: »Happy is he who always looks on the bright side. Who, despite life’s ups and downs, is still guided by good sense!«