Cosi Fan Tutte
In a cafe, Ferrando and Guglielmo (two officers) express certainty that their fiancées (Dorabella and Fiordiligi, respectively) will be eternally faithful. Don Alfonso expresses skepticism and claims that there is no such thing as a faithful woman. He lays a wager with the two officers, claiming he can prove in a day’s time that those two, like all women, are fickle. The wager is accepted: the two officers will pretend to have been called off to war; soon thereafter they will return in disguise and each attempt to seduce the other’s lover.
Act I begins in the Hall of the Gibichungs, a population dwelling by the Rhine. Gunther, lord of the Gibichungs, sits enthroned. His half-brother and chief minister, Hagen, advises him to find a wife for himself and a husband for their sister Gutrune. He suggests Brünnhilde for Gunther’s wife, and Siegfried for Gutrune’s husband. He reminds Gutrune that he has given her a potion that she can use to make Siegfried forget Brünnhilde and fall in love with Gutrune; under its influence, Siegfried will win Brünnhilde for Gunther. Gunther and Gutrune agree enthusiastically with this plan.
The original couple Brünnhilde/Siegfried becomes Brünnhilde/Gunther, Siegfried/Gutrune.