Eisenstein’s house. From outside, we hear Alfred serenading Rosalinda. The Eisenstein’s maid, Adele, enters reading a letter from her sister, Ida, who has been invited to Prince Orlovsky’s that night. If Adele can get a dress, Ida can take her along. Adele bubbles with excitement. But Rosalinda is too preoccupied by Alfred’s serenade to pay attention to Adele’s plea that she be allowed to visit a “sick Aunt.” Besides, Eisenstein is leaving for a five-day prison sentence and must be sent off with a good supper; no one can be spared. When Adele leaves the room, Alfred enters. He has heard of Eisenstein’s departure and plans to return again that evening. Rosalinda is beside herself.

Eisenstein storms in with his lawyer, Dr. Blind. It is Blind who is responsible for this prison affair; and now the term is eight days, not five. In a lively trio, Rosalinda protests her grief – perhaps a shade too much – Eisenstein rages at Blind, who runs through a list of possible legal expedients and leaves. Adele, still in tears about her mythical Aunt, is sent to order a delicious supper for t?e master, and Rosalinda goes to find old clothes for him to wear in prison.

Enter Dr. Falke. A friend of Eisenstein, he has been nursing a grievance against him ever since the last Carnival. Falke, dressed as a bat (hence the title), was left by Eisenstein to find his way in broad daylight in his costume. He has a plan for revenge. Why shouldn’t Eisenstein accept the invitation from Orlovsky, which he has brought? He could go in disguise and give himself up to the authorities in the morning. Rosalinda need never know, nor does Eisenstein guess, that Rosalinda is also involved. The Bat’s revenge is taking shape as Eisenstein accepts.

Rosalinda is astonished that her husband is going to prison in evening clothes. But, disturbed by Alfred’s imminent return, she’ll accept any excuse. For that reason she has given Adele the night off. Rosalinda, Adele and Eisenstein sing a mock-tragic farewell trio; but, their separate anticipations sparkle.

Alfred keeps his promise. Eisenstein is hardly out of the house before his wife’s admirer is eating the supper originally prepared for him. She can’t help noticing that Alfred is beginning to show the effects of the wine he praises. Their song is interrupted when Frank, the new prison governor, appears. He has come to escort Eisenstein to prison. Alfred ropes him into singing but cannot persuade him that he is not Eisenstein. The situation looks compromising; but Rosalinda carries it off with bravado, reconciling Frank to the delay and Alfred to his fate. She fears the worst – Alfred and her husband will meet in prison – but, what can she do? Frank, too, must get to Orlovsky’s. So he hustles Alfred off.

Prince Orlovsky’s house. The party is in full swing. Although too blasé to enjoy parties himself, the Prince likes his own to go well. Woe betide anyone who refuses to drink with him! His song, a mixture of languid nonchalance and adolescent gaucherie, describes him perfectly.

Eisenstein, introduced as Marquis Renard, feels sure he recognizes Adele. Orlovsky and the rest laugh at his mistake. Adele makes fun of him in her soubrette song. Persuaded of his mistake, Eisenstein fails to recognize his wife when she enters as a Hungarian Countess.

She excites his curiosity immediately. Soon, he works his familiar ploy of the chiming watch. He times her heartbeats, but instead of winning her heart, he loses his watch to her, not at all according to plan.

Rosalinda will not unmask and Adele suggests it is because she is not Hungarian. Rosalinda proves her supposed origin by singing the well-known Czardas. Orlovsky, Adele and Eisenstein lead the company in a celebration of Champagne. The general merriment reaches its climax when the Prince urges everyone to dance; this is the famous Fledermaus Waltz. The mutual suspicion of Frank (Chevalier Chagrin) and Eisenstein comes to an end as the clock strikes six and both realize they must get to the prison.

The prison. Frosch, the jailer, has been doing his best to emulate the drinking exploits of Governor Frank. In a word, he is drunk. His inebriated wanderings are occasionally interrupted by a disturbance in Cell No. 12. Alfred is relieving the tedium with snatches of song. Frosch staggers off to curb this nuisance, and Frank enters. He, too, is the worse for wear and fails asleep at his desk. Determined to carry on, Frosch wakes him to deliver his morning report. Eisenstein has been demanding to see his lawyer, who is due immediately. The doorbell rings and Frosch admits Adele and Ida.  Inspired by Frank’s attentions to her last night, Adele hopes he can help her to a stage career. There is another ring and Marquis Renard arrives – Eisenstein has come to turn himself in. He is amused that the Chevalier Chagrin is the prison governor. But Frank is infuriated by his insistence that he, and not the man in the cell, is the real Eisenstein. Rosalinda arrives as Eisenstein is plotting to discover who has taken his place in prison. She hopes to spring Alfred before his presence becomes more embarrassing. But Eisenstein impersonates Dr. Blind and proceeds to cross-examine everybody. In the growing bedlam, the rest of the Orlovsky party arrives, including Falke who finally confesses to the “Revenge of the Bat.” In the final reconciliation, Rosalinda reiterates last night’s drinking song and all join in praise of Champagne.