The incident which gives rise to the opera has taken place before the curtain goes up. Dr. Falke went to a fancy dress ball as a bat. He drank too much and was laid down (still in costume) at the side of the road by his friends, and in the morning has to endure the jeering of local urchins calling out..¨Dr Fledermaus..Dr. Fledermaus!¨ (Dr. Bat..Dr. Bat)!--as he walked ignominiously through the streets. Every since then the sobriquet Dr. Fledermaus has firmly stuck to his coat tails-and the opera pivots on his wish to get even with Eisenstein, chief perpetrator of the joke.
ACT I. Vienna, 1890s.
The Eisenstein home is filled with the serenade of Alfred, a tenor still in love with his old flame Rosalinda, now the wife of Gabriel von Eisenstein. Adele, a chambermaid, saunters in reading an invitation to a masked ball. Rosalinda enters believing she has heard Alfred's voice, but finds only Adele. Adele asks for the evening off to visit a "sick aunt," a plea her mistress dismisses. Alfred enters and begins to woo Rosalinda, who resists but melts on hearing his high A. The suitor leaves as Eisenstein and his lawyer, Dr. Blind, arrive from a session in court. Eisenstein has been sentenced to a fortnight in jail for a civil offense. His friend Falke comes to invite Eisenstein to a masquerade, suggesting he bring along his repeater stop-watch, which charms all the ladies, so he can accumulate pleasant memories to sustain him during his stay in jail. Rosalinda joins Adele in a bittersweet farewell to Eisenstein before he goes off to prison in full evening dress. Sending Adele to her "aunt," Rosalinda receives the ardent Alfred. Their tête-à-tête is interrupted by the warden Frank, who mistakes Alfred for the man he has come to arrest. Rosalinda persuades Alfred to save her name by posing as her husband, and Frank carts him off to jail.
ACT II. At the palace of Prince Orlofsky
Adele, her cousin Ida, and other guests, await the arrival of their host. A quite bored Orlofsky enters — even with Falke's promise of a comedy of errors — and proclaims his guests free to do anything that suits their fancy. "Chacun à son gout." Adele, dressed in one of Rosalinda’s elegant gowns, laughs off Eisenstein's suggestion that she resembles his wife's chambermaid. Frank enters, and Rosalinda arrives disguised as a temperamental Hungarian countess. She is soon wooed by her own husband, whose pocket watch she steals to hold as proof of his philandering. Rosalinda sings about her "native" land, a spirited czardas, after which the guests toast the joys of wine, good fellowship and love. Champagne flows and the guests dance wildly until dawn. When the clock strikes six, Eisenstein staggers off to keep his appointment at the jail.
ACT III. At the prison
Frosch, a drunken jailer, tries to keep order among the inmates who are unable to sleep because of Alfred's singing. Frank arrives, still giddy with champagne, followed shortly by Ida and Adele, who, thinking him a theatrical agent, believes he might further her stage aspirations. Frank, hearing someone at the door, hides the girls in a cell and then admits Eisenstein, who is surprised to learn his cell is already occupied by a man who claims to be Eisenstein and who was found supping with Rosalinda. To obtain an explanation from the impostor, Eisenstein snatches a legal robe and wig from his astonished lawyer. No sooner is he disguised than Rosalinda hurries in to secure Alfred's release and press divorce charges against her errant husband. With her would-be paramour, she confides her flirtation to the "lawyer." Enraged, Eisenstein removes his disguise and accuses his wife of promiscuity, at which Rosalinda whips forth the watch she took from him at the ball. Orlofsky and his guests arrive to celebrate the reconciliation of Rosalinda and Eisenstein, singing a final toast as Eisenstein is taken away.