In the aria “Hier sol lich dich den sehen,” Belmonte expresses his joy at locating the shady nightclub (“The Seraglio”) where his beloved Constanze is being held captive by club owner Pasha Selim. He encounters the club manager Osmin, who is singing a romantic song while working (“Wer ein Liebchen nat beflunden”), which turns into a duet—a confrontation between Belmonte and the uncooperative Osmin. Belmonte’s friend Pedrillo enters and joins Belmonte at the bar and explains the reason for their plight: along with the two women, Constanze and Blonde, he was captured by some “goons” and was turned over to Pasha (who runs a Hollywood-style nightclub that caters to German officers, and has been wooing Constanze). Osmin has expressed an interest in Blonde as well. Belmonte tells Pedrillo that he has someone waiting to help them escape. Pedrillo offers to introduce Belmonte to Pasha as a musician who could play in the band, and gain access to Constanze, the club’s featured singer. In an aria which has become a solo favorite among tenors, “O wie angstlich,” Belmonte describes the anguish he has been through and the anticipation he feels in expecting to see her again. The two men hide as Pasha enters with Constanze and a chorus of waiters, band players and patrons praising their great host. As she heads to the stage, Pasha and Constanze argue: Pasha declares his love for her, while she is reluctant to explain the reason for her refusal. Performing a “torch song” for the club audience (the aria “Ach, ich liebte,”), Constanze sings that she has sworn to be true to another man. In the dialogue that follows, she adds that she might have romantic feelings for Pasha if it weren’t for the fact that she loves another. Constanze exits and Pedrillo introduces Belmonte as a musician, and Pasha offers him a spot in the band, fortunately without an audition. Osmin enters, still trying to keep Belmonte out of the club, and their conflict is expressed in a comic trio with Pedrillo. The scene shifts to the back of the club, where Constanze, in the aria “Weicher Wesche…Traurigkeit,” expresses her dismay at her predicament. Pasha enters and again asks her to accept him, and when she refuses, he becomes threatening. Blonde enters, giving words of encouragement to Constanze, to no avail. Blonde is left alone on stage. Her aria “Durch Zartichkeit und Schmeichein” describes how men should treat women. Osmin has been listening and he tries in vain to win Blonde’s affection, leading to a comic duet between the two, “Ich gehe, doch rate ich dir”. Constanze returns alone, and proclaims how she can withstand any form of torture to remain true to Belmonte in the powerful aria “Martern aller Arten.”
In the club’s back alley, Pedrillo tells Blonde of the escape plan. Recognizing that the overly zealous Osmin poses the most formidable obstacle to their escape, they devise a strategy to drug him. Blonde sings the sprightly aria “Welche Wonne, weiche Lust” expessing her joy at her forthcoming escape, and after she leaves, Pedrillo follows with an attempt to build up his courage in what he realizes will be a risky venture in the aria “Frisch zum Kampfe.” Pedrillo joins Osmin in the empty club to suggest that they should try to be friends, sealing their friendship with a toast. Though he recognizes that Osmin is a “tee-totaller” (non-drinker), he tells him that he would be more fun to be around if he loosened up just a little. Pedrillo slips some sleep medication into Osmin’s glass as they drink a toast to Bacchus (“Vivat Bacchus”), the god of wine. Osmin starts to drift off to sleep, and Pedrillo escorts him offstage. The two pairs of lovers are now united, and in an extended ensemble consisting of two arias sung by Belmonte bracketing a joyous quartet, they declare that love can conquer all. The men momentarily express doubts about the ladies’ faithfulness, but the four join in unison to declare the folly of petty jealousy. Belmonte and Pedrillo are alone on stage, trying out the keys that Belmonte has somehow obtained. Pedrillo, accompanying himself on the mandolin, sings a lullaby to Osmin “Im Mohrenland gefangen war,” a song intended to cue the women that the escape is imminent, but the keys don’t fit, and they decide to use an air vent in the alley instead. There is a lot of physical comedy as the four try to overcome various obstacles. Meanwhile, the sleep medication has worn off, Osmin discovers the escapees and ties Blonde to a chair. Refusing a bribe, Osmin declares that he wants revenge, not money, and in the aria “O, wie will ich triumphieren,” Osmin expresses his joy in anticipating the deaths of his four captives. Having heard the commotion, Pasha enters, and Osmin explains the foiled escape plan. Belmonte desperately explains that he is from a rich family and that his father will gladly pay any ransom that Pasha requests. This plea backfires, as Pasha explains that Belmonte’s father is his worst enemy, a man who stole his wealth and forced him to flee his native land. He asks Belmonte what his father would have done had the roles been reversed, and Belmonte is forced to admit that his father would not show Pasha any mercy. Pasha and Osmin exit to plan the proper punishment for the captives. Belmonte and Constanze proclaim that they are resigned to their impending doom. Pasha returns to inform them of their fate, but before he speaks, Belmonte expresses the hope that his death will pay for the injustices that his father did to Pasha. Pasha explains that he does not wish to repay one injustice with another. He insists that Belmonte tell his father that he was in Pasha’s power and was spared. Pasha tells him that if Belmonte becomes a better man than his father, his mercy will have been rewarded. Pedrillo asks for mercy for himself and Blonde as well, and Pasha grants it, over Osmin’s objection, telling him “If something cannot be won through kindness, it’s better to do without.” In the grand finale, the four former captives praise Pasha’s merciful character individually and collectively. Blonde, however, adds a condemnation of Osmin’s boorish behavior, and he responds with a repetition of his belief that the four should have been killed, but as he rushes out, the four join in articulating a more general principle, that “Nothing is as hateful as revenge. To be generous, merciful, kind and selflessly to forgive is the mark of a noble soul.” The chorus follows with a tribute to Pasha’s greatness as the curtain falls.