The Synopsis
Constanza, a young Spanish woman, together with her English lady-in-waiting Blonde and the latter's fiancé Pedrillo, a servant of Belmonte's have been kidnapped by pirates and sold as slaves to Pasha Selim in Turkey. Belmonte, a Spanish aristocrat and Constanze's fiancé has discovered their whereabouts and has come to free them.
Act One
Belmonte is standing in front of Pasha Selim's palace and longingly awaits his reunion with Constanze. He still has no idea, however, how to bring it off. Belmonte meets the overseer Osmin in the palace garden. Osmin, a servant in the Pasha's employ, is hostile in his behaviour and refuses to provide any information. Osmin then scolds Pedrillo and explains to him that his greatest delight would be to put him to death by a wide variety of methods, "first by decapitation, then by hanging". Pedrillo at last meets up with Belmonte, reporting that the Pasha is still Constanze's unrequited lover and that Osmin is prowling after Blonde. He plans to introduce Belmonte as a foreign architect to the Pasha, who is about to return from a boating excursion with Constanze. Belmonte is beside himself with joy at the prospect of seeing Constanze again.
Pasha Selim and Constanze return from their outing with a large retinue and are welcomed by the Janissaries "singing songs to the great Pasha". Selim asks Constanze why she cannot take pleasure in anything or return his love. She explains to him that she loves someone else and is very sad at being separated from him. The Pasha is adamant and threatens to force her to love him. She must decide before the day is out whether to become his wife - but Constanze would rather die than break her vow - thus leaving the Pasha enchanted at Constanze's firmness of purpose. Pedrillo introduces Belmonte to him as an Italian architect, whereupon he is promptly accepted into the Pasha's service. Belmonte and Pedrillo try to enter the palace, but Osmin bars their path. They push him aside and enter into the building.
Act 2
Osmin is prowling after the pretty Blonde, whom the Pasha has given to him as a slave. Far from being intimidated, she gives him a lesson in etiquette: the way to conquer a good woman's heart is "with tenderness and cajolery", not with "dour commands and squabbling, blows and torments". Osmin tries to threaten her and forbids her to see Pedrillo. Blonde merely laughs at him and threatens that he will "see nothing more if he tarries a moment longer". Constanze is seen, disconsolate and consumed by her longing for Belmonte when she is met by Selim - he asks whether she has decided to become his mistress the next day. Constanze avers that she holds him in honour, but can never love him; nor is she afraid of death: "though tortures of every kind may await me, I scoff at torment and pain".
Pedrillo finds Blonde and informs her that Belmonte is in the palace, he tells her of their escape plans - that Belmonte will come to Constanze's window with a ladder at midnight, and Pedrillo will do the same with Blonde. They will then flee together in a ship lying ready in the harbour. While Blonde takes the happy news to her mistress, Pedrillo makes Osmin drink himself into a stupor, and leads him into the palace. Belmonte and Constanze are reunited at last, but their joy darkens momentarily as the two men ask whether the ladies have been faithful to them. The girls forgive the penitents for their suspicions, and both couples are reconciled.
Act 3
At midnight Pedrillo and Belmonte appear beneath the windows of the seraglio. Belmonte is concerned that their escape may miscarry. Pedrillo gives the girls the pre-arranged signal by singing a serenade, and Belmonte then abducts his lady fair. As Pedrillo tries to do the same with Blonde, Osmin appears, dazed by alcohol. He alarms the guards and has the fugitives arrested. Osmin is elated - he leads the prisoners to Pasha Selim, who is outraged at their treachery. Belmonte tries to convince the Pasha to release him and Constanze for a ransom. When he mentions his true name, Lostados, Selim realizes that Belmonte is the son of his arch-enemy, the man who forced him to flee his country and abandon his beloved and his fortune. He swears his revenge: Both prisoners shall be condemned to die together. They prepare to "leave the world with blissful gaze": Now Pedrillo and Blonde are led into the room, and all await their impending execution. Completely unexpectedly, Selim grants Constanze and Belmonte their freedom, claiming that he shall never act as Belmonte's father had done, that it is for him "a far greater pleasure to repay injustice with beneficence than to eradicate vice with vice".
Pedrillo and Blonde are likewise granted their freedom. Osmin is livid with rage at the thought that Blonde, too, shall go free. Belmonte, Constanze, Pedrillo, and Blonde express their gratitude to Pasha Selim and their admiration for his benevolence. The opera comes to an end in a general hymn of praise to the Pasha: "Long live Pasha Selim!"