Two years prior to the opening scene, Florestan, a nobleman, has been secretly imprisoned by his political rival, the villainous Don Pizarro, in the prison over which he is governor.
In a prison, Marzelline, daughter of the jailer, Rocco, rejects the attentions of her father’s assistant, Jacquino. Her heart is set instead on the new errand boy, Fidelio. However, ‘Fidelio’ is actually Leonore, a noblewoman who has got a job in the prison disguised as a man in order to rescue her husband, Florestan. When Rocco mentions a man lying near death in the vaults below, Leonore, begs Rocco to take her on his rounds, suspecting this might be him.
Don Pizarro, prison governor, learns that Don Fernando, a government minister, is on his way to investigate rumours of cruelty in the prison, so resolves to kill Florestan himself without delay and orders a grave to be dug. Leonore, overhearing his plan, prays for strength to save her husband. She again begs Rocco to let her accompany him to the condemned man’s cell.
Florestan, in the depths of the dungeon, dreams that Leonore arrives to free him. Rocco and Leonore arrive and begin digging the grave, as Don Pizarro has ordered them. Florestan awakens, but does not recognise his wife. Rocco blows on his whistle to signal to Pizarro that all is ready. The governor advances to strike, but Leonore leaps between them, pulls a pistol and reveals her true identity.
At this moment Don Fernando arrives, and Rocco explains to him the murderous plot. Pizarro is arrested, all the prisoners are freed and the reunited Florestan and Leonore rejoice.