Constanza, a young Spanish woman, together with her English lady-in-waiting, Blonde, and the latter’s fiancé, Pedrillo, who is also a servant of Constanza’s Spanish nobleman fiancé, Belmonte, have been kidnapped by pirates and sold into slavery to be purchased by Pasha Selim.
Noble Belmonte tracks them to a seaside palace. While Pasha endeavors to gain favor with Constanza, Pedrillo succeeds in obtaining a position as a gardener. Pasha offers Blonde as a gift to the overseer, Osmin, who attempts to court her.
Belmonte first encounters Osmin, who initially acts civil until Belmonte asks after Pedrillo. Feeling threatened that Pedrillo wants to take Blonde from him, Osmin ushers Belmonte to leave. Pedrillo enters, wishing to make amends, but Osmin rejects his attempt.
Belmonte returns to find his former servant, who tells him that although the Pasha loves Constanza, he resists forcing himself on her. He wishes to seduce her with charm rather than resort to force. Belmonte and Pedrillo scheme to arrange a meeting between Constanze and Belmonte; they plan to escape by boat, the four of them, if they can outmaneuver Osmin. Belmonte is heart sick away from his love, pining to be with her again. He sees Constanza entering with Pasha Selim. Pasha inquires why she resists his flirtations and she admits in honesty that she cannot forget the love for her fiancé from whom she was separated.
After Constanza exits, Pedrillo introduces Belmonte to Pasha as a promising young architect. Pasha welcomes him and, departing, arranges a conference for the next day. Osmin bars the way when Belmonte and Pedrillo try to enter the palace, but he is confused easily, and the two foreigners march him around in circles. Disoriented, Osmin does not notice they have gained access.
In a garden, Blonde confounds Osmin with her cleverness. Constanza complains of her sad state, which does not improve when Pasha persists in asking her to marry him. She proudly refuses, preferring torture, even death. When they have gone, Blonde and Pedrillo dance into the garden, discussing their plan of escape: they will deceive Osmin with alcohol and all four lovers will leave on Belmonte’s ship. Later, Pedrillo goes about his business, finding Osmin cooperative, though drinking wine is against his morals. Thoroughly inebriated, the fat man wanders away with the bottle, leaving the coast clear for Belmonte to meet Constanze. Their reunion is shared by Blonde and Pedrillo.
Just before midnight, Pedrillo sings a serenade, the signal for escape. But he accidentally wakes Osmin, who has recovered from being hung over and takes them all to Pasha. Belmonte suggests Pasha Selim demand a handsome ransom from Belmonte’s wealthy family, the Lostados. At this, Pasha realizes that Belmonte is the son of an old enemy who exiled Pasha from his home country. Eventually, Pasha decides that rather than take blood for blood, he will repay evil with good, freeing Constanza and Belmonte, (and Blonde and Pedrillo). This does not sit well with Osmin, who will lose Blonde, but he is promised other rewards. The grateful lovers praise their Pasha Selim as they prepare to set sail.