Florestan disappeared two years ago. His wife Leonore suspects that he has been imprisoned in a secret dungeon because he defied the governor Don Pizarro who has established a system of corruption and despotic violence. Disguised as a man and using the alias “Fidelio” she works as assistant to the gaoler Rocco in a state prison. Rocco’s daughter Marzelline has fallen in love with the intelligent youth, abandoning her previous betrothed, Jaquino. Although Leonore is unhappy about deceiving the girl, she grows ever more certain that the mysterious inmate, whom only Rocco is permitted to see, is indeed Florestan. Suddenly Pizarro arrives and learns of an impending prison inspection. He now urgently wants to eliminate his enemy. Rocco and Fidelio dig a grave in the underground prison. It emerges that the prisoner really is Florestan. When Pizarro comes to kill him, Leonore leaps between them. In this moment the minister arrives and realises what Pizarro has been up to and punishes him. Florestan and Pizarro’s other victims are set free.