Two years prior to the opening scene, the Spanish nobleman Florestan has exposed or attempted to expose certain crimes of a rival nobleman, Pizarro. In revenge, Pizarro has secretly imprisoned Florestan in the prison over which he is governor. Simultaneously, Pizarro has spread false rumours about Florestan's death. The warden of the prison, Rocco, has a daughter, Marzelline, and an assistant, Jaquino, who is in love with Marzelline. The faithful wife of Florestan, Leonore, suspects that her husband is still alive. Disguised as a boy, under the alias 'Fidelio', she gains employment working for Rocco. As the boy Fidelio, she earns the favour of her employer, Rocco, and also the affections of his daughter Marzelline, much to Jaquino's chagrin. On orders, Rocco has been giving the imprisoned Florestan diminishing rations until he is nearly starved to death.