Gabriel von Eisenstein has been sentenced to eight days in prison for insulting an official, partially due to the incompetence of his attorney, Dr. Blind. Adele, Eisenstein's maid, receives a letter from her sister, who is in the company of the ballet, inviting her to Prince Orlofsky's ball. She pretends the letter says that her aunt is very sick, and asks for a leave of absence ("Da schreibt meine Schwester Ida"/"My sister Ida writes to me"). Falke, Eisenstein's friend, arrives to invite him to the ball (Duet: "Kommt mit mir zum Souper"/"Come with me to the souper"). Eisenstein bids farewell to Adele and his wife Rosalinde, pretending he is going to prison (Trio: "O Gott, wie rührt mich dies!"/"Oh dear, oh dear, how sorry I am") but really intending to postpone jail for one day and have fun at the ball.
After Eisenstein leaves, Rosalinde is visited by her former lover, the singing teacher Alfred, who serenades her ("Täubchen, das entflattert ist"/"Dove that has escaped"). Frank, the governor of the prison, arrives to take Eisenstein to jail, and finds Alfred instead. In order not to compromise Rosalinde, Alfred agrees to pretend to be Eisenstein and to accompany Frank. (Finale, drinking song: "Glücklich ist, wer vergisst"/"Happy is he who forgets" followed by Rosalinde’s defence when Frank arrives: "Mit mir so spät im tête-à-tête"/"In tête-à-tête with me so late," and Frank’s invitation: "Mein schönes, großes Vogelhaus"/"My beautiful, large bird-cage.")
A summer house in the Villa Orlofsky
It transpires that Falke, with Prince Orlofsky's permission, is using the ball as a way of getting revenge on Eisenstein. The previous winter, Eisenstein had abandoned a drunken Falke dressed as a bat (and thus explaining the opera's title) in the center of town, exposing him to ridicule the next day. As part of his scheme, Falke has invited Frank, Adele, and Rosalinde to the ball as well. Rosalinde pretends to be a Hungarian countess, Eisenstein goes by the name "Marquis Renard," Frank is "Chevalier Chagrin," and Adele pretends she is an actress.
The ball is in progress (Chorus: "Ein Souper heut' uns winkt"/"A souper is before us") and the Prince welcomes his guests ("Ich lade gern mir Gäste ein"/"I love to invite my friends"). Eisenstein is introduced to Adele, but is confused as to who she really is because of her striking resemblance to his maid. ("Mein Herr Marquis"/"My lord marquis," sometimes referred to as "Adele's Laughing Song").
Then Falke introduces the disguised Rosalinde to Eisenstein (Csárdás and she convinces all the she is Hungarian by singing the cardas: "Klänge der Heimat"/"Sounds from home"). During an amorous tête-à-tête, she succeeds in extracting a valuable watch from her husband's pocket, something which she can use in the future as evidence of his impropriety. (Watch duet: "Ach, wie wird mein Auge trübe"/"My eyes will soon be dim"). In a rousing finale, Orlofsky makes a toast to champagne, the company celebrates (The Drinking song: "Im Feuerstrom der Reben"/"In the fire stream of the grape"; followed by the canon: "Brüderlein, Brüderlein und Schwesterlein"/"Brothers, brothers and sisters" and the waltz finale, "Ha, welch ein Fest, welche Nacht voll Freud'!"/"Ha, what joy, what a night of delight.") Eisenstein and Frank dash off as the clock strikes six in the morning.
In the prison offices of Governor Frank
The next morning they all find themselves at the prison where the confusion increases and is compounded by the jailer, Frosch, who has profited by the absence of the prison director to become gloriously drunk.
Adele arrives to obtain the assistance of the Chevalier Chagrin (Melodrama; Couplet of Adele: "Spiel' ich die Unschuld vom Lande"/"If I play the innocent peasant maid") while Alfred wants nothing more than to get out of jail. Knowing of Eisenstein's trickery, Rosalinde wants to begin an action for divorce, and Frank is still intoxicated.
Frosch locks up Adele and her sister Ida, and the height of the tumult arrives when Falke appears with all the guests of the ball and declares the whole thing is an act of vengeance for the "Fledermaus". (Trio between Rosalinde, Eisenstein, Alfred: "Ein seltsam Abenteuer"/"A strange adventure"). Everything is amicably arranged (with Eisenstein blaming the intoxicating effects of champagne for his act of infidelity and Frank volunteering to support Adele's artistic career), but Eisenstein is compelled to serve his full term in jail (Finale, "O Fledermaus, o Fledermaus"/"Oh bat, oh bat, at last let thy victim escape").