Jaquino, the prison gatekeeper, is wooing Marzelline, who is the daughter of Rocco, the head jailer. Jaquino is having little success; Marzelline refuses his advances because she loves Fidelio, Rocco’s new assistant. Fidelio, however, is really a woman named Leonore; she has disguised herself and is looking for her husband, Florestan, who was imprisoned two years earlier for his political views. Rocco, pleased with Fidelio’s cleverness and skills, announces that Fidelio will marry Marzelline in a few days.
Continuing her search for her husband, Leonore offers to help Rocco with the lower prison cells, an area forbidden to her. The governor Don Pizarro must approve such a request, says Rocco, and even so there would still be one cell Fidelio never can enter. Marzelline asks if that is the location of the “special prisoner” that Rocco has mentioned. Upon learning that the prisoner has been there for two years, Leonore realizes it could be her husband.
Don Pizarro receives a letter alerting him to a visit by the Minister of State Don Fernando, who is coming to investigate charges that several victims of political tyranny are jailed in the dungeons. Distressed by the possibility that Fernando will discover Florestan (whom Fernando thinks died two years ago), Pizarro orders Rocco to murder Florestan. When Rocco refuses, Pizarro orders him to dig a grave while he himself kills the prisoner. Overhearing the plot, Leonore denounces Pizarro’s treachery and resolves to save her husband.
When Rocco returns from his discussion with Pizarro, Leonore persuades him 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.
Having gained Pizarro’s consent, Rocco asks Leonore to accompany him to the forbidden cell to dig a grave. 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 he kills Florestan. Pizarro orders the prisoners locked up, however, and urges Rocco to dig the grave.
--- INTERMISSION ---
Alone in the dark, the half-starved Florestan is despondent, but he accepts his misfortune because he stood against injustice. For a moment, he imagines Leonore as an angel, leading him to heavenly freedom, and then he falls back exhausted.
Rocco and Leonore enter the cell, and at first Leonore does not recognize the prisoner. When he speaks, however, she realizes that it is her husband. Florestan realizes his death is imminent. 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, exclaiming, “First kill his wife!” She declares that she will not let him die, no matter how powerful Pizarro may be. Pizarro decides to kill them both, but she forces him back. Suddenly, trumpets signal the arrival of Don Fernando. Jaquino enters with guards to escort Pizarro away. Overcome with joy and relief, Leonore and Florestan embrace.
Outside, people fill the prison courtyard as Fernando, Pizarro, and the prisoners march in. The crowd hails Fernando as an emissary of their enlightened leader, releasing them 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 punishment and he is arrested. Fernando gives Leonore a key so she can remove Florestan’s chains, and the crowd salutes the depth of her courage and love.