Florestan disappeared two years ago and is secretly being incarcerated by his enemy Don Pizarro. Florestan's wife, Leonore, disguises herself as a young man, Fidelio, and enters the service of the governor at the prison where her husband is being held.

ACT I: (The prison courtyard on a spring morning)

Jaquino, the turnkey, and Marzelline, the jailer's daughter, are alone. Jaquino asks when she will agree to marry him, but she says that she will never marry him now that she has fallen in love with Fidelio. Rocco, the jailer, enters with Fidelio, who is carrying a heavy load of newly-repaired chains. Rocco compliments Fidelio's skill and offers Marzelline's hand in marriage. Rocco tells them, however, that unless they have money, they will never be happy. Fidelio begs Rocco, as a mark of trust, to let him help in the cells. He agrees but explains that there is one cell where Fidelio will never be allowed to go: the cell of the unnamed prisoner.

Pizarro arrives with an armed escort and is given a letter telling him that the Minister, Don Fernando, has heard rumors of injustice and is on his way to inspect the prison. Pizarro decides he must kill Florestan, the unnamed prisoner, or be found out. He posts a lookout trumpeter and tells Rocco to prepare Florestan's grave.

Fidelio, hoping to find Florestan, persuades Rocco to let the prisoners roam in the garden and enjoy the beautiful weather. Rocco reenters and tells Fidelio that he will be allowed to help dig the grave. Leonore weeps knowing this must be her husband's grave, but knows it is her duty as Fidelio to help Rocco. Pizarro returns and angrily orders the prisoners back to their cells.

ACT II: (A deep dungeon)

Florestan is alone in his cell. He is chained to a stone, but his spirit is unbroken. He has a vision of Leonore coming to save him and tries to follow her, but the chain drags him back and he collapses.

Rocco and Fidelio enter to dig the grave and find Florestan asleep, his face hidden. Leonore resolves to save the prisoner, whoever he is. Florestan awakes and is recognized instantly by Fidelio, who convinces Rocco to disobey orders and give Florestan food and wine. Suddenly Pizarro enters and is about to stab Florestan, when Leonore springs between the two enemies, shielding her husband. She cries out "First, kill his wife!" as she pulls a pistol on Pizarro. At that moment, the trumpeter sounds; the minister has arrived.

Leonore and Florestan thank God at their reunion. Led Florestan, the released prisoners and the people join in a hymn to praise Leonore, the noble woman who saved her husband's life.