Belmonte, a Spanish nobleman, has made his way to Turkey in search of Constanze, to whom he is betrothed. After being captured by pirates, Constanze, her English maid Blonde, and Pedrillo, servant to Belmonte, have all three been bought by the Pasha Selim and taken to his house on the seacoast of Turkey. The Pasha has made Constanze the favorite of his harem, despite the fact that she refuses to return his love. He employs Pedrillo as his gardener and has made a present of Blonde to his chief steward, Osmin. On his arrival at the house, Belmonte is refused admission by Osmin. Shortly afterwards Belmonte encounters Pedrillo who tells him all that has happened and proposes to introduce Belmonte to the Pasha as a famous architect. The Pasha enters with Constanze and pleads with her to marry him. She refuses, having sworn eternal fidelity to Belmonte. The Pasha gives her one day in which to change her mind. Pedrillo presents Belmonte to the Pasha, who courteously welcomes him. As Pedrillo leads Belmonte towards the house, Osmin tries to prevent them from entering, but without success.

Blonde and Osmin have a difference of opinion about Blonde’s role as a slave, Blonde expressing the English, Osmin the Turkish point of view. She finally chases Osmin away. The Pasha again presses his claims on Constanze but she still refuses to give in to him in spite of his threats to have her tortured. Pedrillo tells Blonde that Belmonte has arrived and is standing by, ready to rescue them by ship at midnight. Pedrillo invites Osmin to drink with him, having previously mixed a sleeping draught with the wine. Osmin overcomes his scruples about taking wine, drinks deeply and is pushed off to bed. Constanze and Belmonte are brought together by Blonde and Pedrillo.

Pedrillo serenades the ladies as their cue for escape. Osmin, roused by the guards, enters as the lovers are in the act of escaping. The lovers are arrested and brought before the Pasha. When the Pasha discovers that Belmonte is the son of a man who once did him great wrong, he prepares to have all four lovers tortured and put to death. Later he forgives them and, to Osmin’s rage, magnanimously grants them their freedom.