Eisenstein’s house. From outside, we hear Alfred serenading Rosalinda. He calls her his dove and recalls their love. The Eisenstein’s maid, Adele, enters reading a letter from her sister, Ida, a member of the Ballet, who has been invited to Prince Orlovsky’s that very night. If Adele can get a dress, Ida can take her along. Adele fairly bubbles with excitement. But Rosalinda is far too preoccupied by Alfred’s serenade to pay much attention to Adele’s plea that she be allowed to visit a “sick Aunt”. Besides, Eisenstein is leaving to begin a five-day prison sentence. He must be sent off with a good supper and no one can be spared. The minute 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. There is a lively trio in which Rosalinda protests her grief – perhaps a shade too much – Eisenstein rages at Blind, and the lawyer runs through a list of possible legal expedients. The lawyer leaves. Adele, still in tears about her mythical Aunt, is sent off to order a delicious supper for the master, and Rosalinda goes to find some old clothes for him to wear in prison.

Enter Dr. Falke. A friend of Eisenstein, he has, we learn, been nursing a grievance against him ever since the last Carnival. It seems that Falke, dressed as a Bat (hence the title), was left by Eisenstein to find his way in broad daylight in this unconventional costume. He has a plan for revenge. Why, he says, 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 invited. The Bat’s revenge is taking shape as Eisenstein accepts.

Rosalinda is astonished to hear that her husband is going to prison in evening clothes. But, still disturbed by Alfred’s imminent return, she’ll accept any excuse. For that reason, also, 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 her companion 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 Herr von 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. Does the governor think she would be dining this late with a man not her husband? Enchantingly, she manages to reconcile 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 his wife’s maid, Adele. Orlovsky and the rest laugh at him for his curious mistake. Adele takes the opportunity to make fun of him in her soubrette song. Persuaded of his mistake, Eisenstein fails to recognize his own wife when she enters as a Hungarian Countess.

She excites his curiosity immediately. Soon, he is working his familiar (to her) 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 that it is because she is not Hungarian. Rosalinda proves her supposed origin by singing the well-known Czardas. Immediately, Orlovsky, Adele and Eisenstein, in turn, 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, which has continued throughout the evening, 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 of incarceration with snatches of song. Frosch staggers off to make another attempt to curb this nuisance, and Frank enters. He, too, is the worse for wear and falls asleep at his desk. But, determined to carry on, Frosch wakes him to deliver his morning report. Herr Eisenstein has been restlessly demanding to see his lawyer and Dr. Blind is due almost immediately. The doorbell rings and Frosch admits Adele and Ida. Inspired by his 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 the praise of Champagne.