Florestan has been unjustly imprisoned and silenced by his enemy, Pizarro, the governor of a state prison used to detain political prisoners. There he is slowly being tortured and starved to death. Florestan’s wife, Leonora, believing her husband to still be alive, makes it her mission to find and free him. Leonora, disguised as a young man named “Fidelio,” she is employed by the chief jailer, Rocco, as his assistant.
The young prison employee Jaquino courts Marzelline in vain, for she has fallen in love with Fidelio. Her father, Rocco, also wants a union between his daughter and Fidelio and hopes for the governor’s permission to use the latter as a helper with the secret prisoners. Marzelline fears that Fidelio won’t be able to bear all the misery that such work entails, but Leonore knows she must have courage and strength to carry out her secret plan—the rescue of her husband.
Pizarro receives news from a friend that the minister, Don Fernando, intends a surprise inspection of the prison. Fearing that Florestan will be found, he resolves to have him killed. A soldier is posted on the guard tower to give a signal as soon as the minister is sighted. Rocco, while not willing to be a murderer, agrees to hold his tongue for money and later hide Florestan’s body in a ruined cistern. Leonore, who has overheard the plan to murder a prisoner, resolves to save him, whoever he may be. At her request Rocco allows some of the prisoners to go into the courtyard. Leonore is distressed that Florestan is not among them. Pizarro, furious at Rocco’s independent actions, has the prisoners locked up again.
In a secret isolated cell, Florestan, weakened from torture, hunger and thirst, has a vision: his wife appears to him as an angel of freedom. Rocco and Leonore come down into the deepest vault of the prison to open the cistern which is to be used as a grave. Leonore recognizes the unknown prisoner as her husband. Against Pizarro’s orders she hands him bread and wine, but dares do no more. When Pizarro appears and tries to stab the defenseless Florestan, she rushes to shield him. Pizarro, in a burst of rage, attempts to kill them both. Leonore draws a pistol and levels it at him. Suddenly a trumpet call is heard announcing the minister’s arrival. Leonore and Florestan are saved and reunited.
Florestan’s fellow prisoners have been freed by the minister and Leonore removes Florestan’s chains. Pizarro is arrested and led away, as the crowd hails Leonore for her courageous actions.