Act 1
Jaquino is wooing Marzelline, with little success; Marzelline refuses his advances because she loves young Fidelio, her father Rocco’s new assistant. Fidelio, however, is really a woman named Leonore; she disguised herself the better to search for her husband, Florestan, who was imprisoned two years earlier for his political views. Rocco, pleased with Fidelio’s diligence and ingenuity, gives his blessing to the wedding of Fidelio and Marzelline.
Leonore offers to help Rocco with the restricted lower cells of the prison. The governor Don Pizarro must approve such a request, says Rocco, and even if he did there would still be one cell Fidelio could never enter. Marzelline asks if that is the location of the “special prisoner” she’s heard Rocco mention. Upon learning that the prisoner has been there for two years, Leonore realizes it could be her husband.
Don Pizarro arrives with soldiers. He is handed a letter alerting him to a visit by the Minister of State, Don Fernando, who is coming to investigate charges that Pizarro is holding political prisoners. Distressed by the possibility that Fernando will discover Florestan (whom Fernando thinks died two years ago), Pizarro vents his fear and anger in the aria “Ha! Welch ein Augenblick,” then orders Rocco to kill Florestan. But Rocco—who ordinarily is easily bullied into doing Pizarro’s dirty work for him—draws the line at murder. So Pizarro orders him to dig a grave while he kills the prisoner himself. Leonore, who has overheard, denounces Pizarro and resolves to save her husband in the aria “Abscheulicher!”
Leonore and Marzelline persuade Rocco to let the prisoners walk in the yard. Blinded by daylight, the prisoners enjoy a moment of freedom as Leonore looks for Florestan among them ("Prisoners’ Chorus").
Rocco, having gained Pizarro’s consent, asks Leonore to accompany him to the forbidden cell. Marzelline and Jaquino enter with the news that Pizarro is in a rage about the prisoners’ walk in the yard. Pizarro enters and confronts Rocco, who explains that this is a diversion while Florestan is killed. Pizarro orders the prisoners locked up and tells Rocco to meet him downstairs.
Act 2
Florestan, near despair and death, accepts his misfortune because he stood against injustice. At the climax of his aria “Gott! Welch Dunkel hier,” he imagines Leonore as an angel, leading him to heavenly freedom, and then falls back exhausted.
Rocco and Leonore enter the cell. At first Leonore does not recognize the prisoner. When she hears his voice, however, she realizes it is her husband. Pizarro enters and identifies himself as the man Florestan had sought to overthrow years ago. As Pizarro moves to murder Florestan, Leonore rushes between them, crying out, “First kill his wife!” There is a struggle; but suddenly a trumpet call signals the arrival of Don Fernando. Jaquino enters with guards to escort Pizarro away. Overcome with joy and relief, Leonore and Florestan embrace during the duet “O namenlose Freude!”
Outside, people fill the prison courtyard. The crowd hails Fernando as an emissary of their enlightened leader, releasing the prisoners from unjust imprisonment. Rocco presents Leonore and Florestan to Fernando, who is astonished that his friend is alive. After hearing of Florestan’s misfortunes and Leonore’s heroism, the crowd calls for Pizarro’s immediate arrest. Fernando gives Leonore a key so she can remove Florestan’s chains, and the crowd salutes the depth of her courage and love.