Constanze, Blonde (her English lady-in-waiting), and Pedrillo (Blonde’s fiancé and servant to Constanze’s fiancé, Belmonte) have all been kidnapped by pirates and sold into captivity to Pasha Selim. The Pasha endeavors to gain favor with Constanze, and Pedrillo obtains a position as a gardener. The Pasha later offers Blonde as a gift to his irascible overseer, Osmin, who attempts to court her.
Noble Belmonte (Constanze’s fiancé) tracks the three to a seaside palace. He encounters Osmin and inquires about Pedrillo. Osmin is threatened by Pedrillo’s pursuit of Blonde and ushers Belmonte to leave. Pedrillo then enters and attempts to make amends, but is rejected by Osmin.
Belmonte returns to find Pedrillo, who tells him of the Pasha’s love for Constanze. He explains that the Pasha wants to seduce her with his charm. Belmonte is heart sick; he and Pedrillo scheme to outmaneuver Osmin, meet their beloved ones, and escape by boat.
Belmonte spies Constanze entering with the Pasha. The Pasha asks why she resists his flirtations, and she admits she cannot forget her love for her fiancé, Belmonte. Pedrillo begins to execute the escape plan by disguising Belmonte as a young architect. When Belmonte and Pedrillo encounter Osmin, he tries to bar their entrance. He is easily confused by the foreigners’ distracting march and does not notice as Belmonte and Pedrillo steal into the palace.
In a garden, Blonde confounds Osmin with her cleverness and Constanze is saddened by the Pasha’s constant advances. She proudly rejects his marriage proposal, preferring torture, even death. After they exit, Blonde and Pedrillo dance into the garden, discussing their escape plan: to distract Osmin with alcohol and flee on Belmonte’s ship.
Despite Osmin’s protest, Pedrillo successfully convinces him to guzzle wine. Thoroughly inebriated, Osmin wanders away with the bottle, leaving a clear path for all the lovers to reunite.
Just before midnight, Pedrillo signals the escape by singing a serenade, accidentally waking Osmin. The four are taken to the Pasha. Scared and hopeless, Belmonte offers the Pasha a handsome ransom from his noble family for their freedom. During the negotiation, the Pasha discovers that Belmonte is the son of an old enemy who exiled him from his home county. Eventually, the Pasha mercifully liberates them all, disappointing Osmin. The grateful lovers praise the Pasha’s benevolence as they prepare to set sail.