Konstanze, Blonde and Pedrillo have been abducted and imprisoned in the seraglio of Pasha Selim. Belmonte has set out to rescue Konstanze, his betrothed.
Belmonte has found his way to the seraglio where Konstanze, Blonde and Pedrillo are imprisoned. He is confronted by Osmin, overseer of the Pasha’s palace, who refuses to answer his enquiries. Pedrillo tells Belmonte how they have been treated in the seraglio. They decide that Belmonte will enter into Selim’s service to prepare their escape. Selim seeks to gain Konstanze’s love, but her thoughts are all with Belmonte. Belmonte is hired as Selim’s servant. The mistrusting Osmin is outraged.
Osmin has his eye on Blonde, who tells him in no uncertain terms what she expects of men. They argue. Konstanze is in despair. She has nevertheless decided not to give in to Selim, even though it might have violent consequences.
Meanwhile, Blonde has heard news of a plan to escape and openly rejoices, but Pedrillo is terrified by the very thought of the inherent dangers. He therefore gets Osmin drunk to put him out of action. Belmonte hopes their efforts to escape will succeed. Konstanze, Belmonte, Blonde and Pedrillo meet to discuss the plan for their escape. The women manage to quell the men’s suspicions that their loved ones have been unfaithful.
A song from Pedrillo gives the women the agreed sign for the escape, but Osmin intercepts all four and locks them up. Their attempt to escape has failed. Belmonte tries to persuade Selim to set them free in return for a ransom. It turns out that Belmonte’s father is Selim’s arch-enemy. Selim, in a furious rage, informs the prisoners that they will shortly be executed. Osmin is triumphant. Belmonte and Konstanze comfort each other and prepare to face death together. Selim returns and frees the four prisoners, for, “Those whom one cannot win over by kindness one has to release.” Selim remains behind with Osmin in his palace.