Alfred sings a serenade to Rosalinde, his old flame. The maid, Adele, has received an invitation to a gala ball at the house of Prince Orlofsky and tries, unsuccessfully, to convince her mistress to give her the night off. Eisenstein appears enraged, accompanied by his attorney Dr. Blind; instead of five days, he will have to spend eight days in prison. Soon he learns from his old friend, Dr. Falke, he has been given one more evening reprieve in which he can also attend the Prince’s ball. His departure plays into the hands of the bold lover, Alfred, who promptly usurps Eisenstein’s position as master of the house. Frank, the prison warden, now bursts onto the scene to fetch Eisenstein personally. To prevent Rosalinde from being compromised, Alfred pretends to be Eisenstein and is carried off.

ACT II:  Prince Orlofsky’s garden palace.

Spirits are high as Dr. Falke promises Orlofsky the comedy of a lifetime. One after another they make their appearances: Adele disguised as an actress, Eisenstein as the Marquis Renard, Frank as the Chevalier Chagrin, Rosalinde as a fiery Hungarian countess. Stage one of the revenge begins: Eisenstein addresses Adele as his maid and is rebuked for his poor breeding. Stage two: he forms a friendship with, of all people, Chagrin, the prison warden. Stage three: he makes love to his own wife. The revels reach a climax and the clock strikes six. Eisenstein must go off to prison and Frank must return to work.

ACT III:  The municipal prison.

The drunken jailer, Frosch, gropes in the dark. Frank, no less drunk, returns. Two ladies appear: Ida and Adele, seeking the protection of the Chevalier Chagrin to advance their stage careers. Eisenstein arrives and has difficulty identifying Chagrin as the prison warden – much less establishing his own identity. He is told, to his astonishment, that a Herr Eisenstein had already entered his cell the day before. Rosalinde rushes in to liberate Alfred and to upbraid her husband. Eisenstein is livid with rage. Finally, the revelers arrive in full force and all is explained: the champagne is to blame for everything.

http://www.su.edu/suopera/Performances/Fledermaus/main/synopsis.htm