The Eisensteins’ Apartment: Alfred serenades his old flame Rosalinda, now married to Gabriel von Eisenstein. Adele, Rosalinda’s maid, has been invited to a ball hosted by the fantastically wealthy Prince Orlovsky, but Rosalinda refuses to let her have the night off. Eisenstein and his lawyer, Blind, arrive: Eisenstein must begin a jail sentence that night. Eisenstein’s friend Falke urges him to delay going to jail until morning and to join him at the ball. While Eisenstein changes, Falke sets in motion a plot to pay back a prank played on him by Eisenstein when they were both young men. It involved leaving Falke drunk in the town square, wearing a bat costume (the “Fledermaus“ of the title). Falke invites Rosalinda to the ball, telling her that if she comes in disguise, she’ll be able to observe her husband flirting with other women. Rosalinda agrees to attend, and gives Adele the night off after all.

Prince Orlovsky’s Villa: Prince Orlovsky doubts that Falke’s promised entertainment will brighten his spirits, but proclaims that his guests should behave however they like. Adele arrives, and her sister Ida decides to present her as a Russian actress, “Olga.“ Eisenstein and Prison Warden Frank enter, posing as Frenchmen, per Falke’s instructions. Finally, Rosalinda arrives, disguised as a Hungarian countess. The crowd sings, dances, toasts, and flirts until the clock strikes six. Eisenstein rushes off to jail.

Jail: Frosch the jailer is vexed by the late arrival of his boss, Frank, who finally appears, tipsy and enraptured by memories of his magical evening. They are joined, in rapid succession, by each of the other players in Falke’s grand joke, now finally reaching its punchline.