Konstanze, a young Spanish lady, Blonde, her English maid and Pedrillo, her fiancé’s servant were taken prisoners by pirates, who subsequently sold them as slaves.  They were bought by Pasha Selim, a Muslim man of Spanish origin who renounced his former faith, converted to Islam and made a fortune in Turkey. He has fallen in love with Konstanze and has made her his favourite. The three hostages are being held in “courteous captivity” in the palace he has by the sea. Belmonte, Kostanze’s fiancé, alerted by the letters that his servant Pedrillo has managed to smuggle out, has followed his indications to sail from Spain to the palace in order to release them.

The first act opens with Belmonte, who has just arrived in Turkey, contemplating with concern the walls behind which he knows his fiancée is being held prisoner, while Osmin, the palace overseer, meditates about love and about how to ensure the beloved’s faithfulness by resorting to coercion, if necessary. Belmonte’s curiosity and his numerous questions about Pedrillo, who now works at the palace as a gardener, end up raising Osmin’s suspicions. Moreover, the leniency shown by Selim, his master, towards Pedrillo awakens in him the desire to subject him to endless torture. Belmonte and Pedrillo manage to meet but, while the servant is telling his master about the difficulties that his plan to rescue them will involve, Belmonte´s major concern is to find out whether his fiancée has been faithful during her captivity.

His impatience and unrest grow, especially when Selim and Konstanze disembark after a pleasure trip at sea. Selim, who has been respecting Konstanze, presses her to give herself to him, but the young woman sings a lament for her fiancé, from whom she was forced to separate. This confession inflames the Pasha’s feelings even more and he gives her one day more to change her mind. Taking advantage of Selim’s well-known interest in architecture, Pedrillo manages to bring his master into the palace by pretending he is an architect who wants to work at the palace. Selim invites him to stay so that they can talk later on. Osmin enters and tries to bar their way, but the two men push him aside and manage to enter the palace.

In the second act, Osmin tries to force Blonde to love him. Even though she claims that only tenderness and freedom can inspire love, Osmin defends the use of force. The argument heats up and Osmin gets really angry when he realizes Blonde’s strength, as she courageously threatens and defies him. In the meantime, Konstanze cries for her past happiness now that the Pasha’s deadline is approaching. She seems determined and says she would prefer death to betraying her beloved, even though Selim, confused, threatens to torture her.

Pedrillo informs Blonde about the escape plans devised by Belmonte and about his own strategy to avoid Osmin’s surveillance and be able to run away: he has to (and does) convince this Muhammad follower to drink in honour of Bacchus some wine in which Pedrillo will have previously poured a sleep-inducing drug. In spite of the promise that their escape will have a happy ending, Belmonte cannot help suspecting that Konstanze may have fallen in love with the Pasha, whereas Pedrillo wonders if Osmin has forced Blonde to give herself to him. The two women get annoyed at their suspicions of unfaithfulness but they apologize and there is joy and peace between both couples again.

The third act starts at midnight. Everything is ready for the escape and Belmonte reflects on the power of love. Pedrillo will sing a serenade, which will tell both women that the coast is clear. But this wakes Osmin once Konstanze has gone down through her window and she and Belmonte have headed for the port. The guards take the four fugitives before the Pasha, while Osmin tastes his imminent revenge.

Before Selim, who is hurt for their ingratitude, Konstanze and Belmonte beg for mercy. Belmonte tries to take advantage of his belonging to a noble family, the Lostados. But the Pasha tells him that his father, the military governor of Oran, is his sworn enemy, since he forced him to exile from his country. This unexpected revelation brings Belmonte down, since he is now to blame for causing the inexorable death of the four hostages. While they await the end of their lives, Konstanze and Belmonte comfort each other. But when Selim delivers his judgment, he is fair and magnanimous, as he does not want to repeat his rival’s evil behaviour. In the subsequent praise chant, celebrating Selim’s nobility, only Osmin’s anger and rage strike the discordant note. The choir of Janissaries proclaims, to conclude, the Pasha’s glory and humanity.