Act I

Ferrando is in love with Dorabella and Guglielmo is in love with Fiordiligi, her sister. Don Alfonso outrages the men by stating that the girls will sooner or later be unfaithful to them; he makes a bet with them that he can prove his words within the space of a day, but that Ferrando and Guglielmo must follow his orders completely during that time. Dorabella and Fiordiligi are waiting impatiently and longingly for their lovers. Alfonso, however, arrives instead and imparts the disastrous news that their fiances must leave immediately for the battlefield. The couples swear eternal fidelity and with great difficulty the sisters bid farewell to their lovers. Ferrando and Guglielmo leave for the front.

Dorabella cannot restrain her despair. The servant girl Despina reacts matter-of-factly and advises the sisters to look for new lovers. Alfonso decides to involve Despina partially in his plans. He introduces her to two exotic foreigners whom he says are in love with Fiordiligi and Dorabella: Despina’s job is to help them obtain their desires. The men’s disguise is complete, for Despina does not recognise them. The sisters are horrified that strange men have gained access to their house. Fiordiligi is offended to the core by their shameless courting and proclaims the steadfastness of her and Dorabella’s fidelity.