Duke Ubaldo of (fictitious) Roccamontana wants to overpower neighboring Italian states by concluding an alliance with the mighty Spaniards. The barons of his council are frightened or bribed to agree, except two. The old count of Archidosso is killed, his daughter Laura told her hand was promised to the duke. The other, count Riccardo d'Argentari, is beaten up and thrown in the river, presumed dead. But friends save him, so he can become the masked 'black captain' of a rebellion movement. Ubaldo's equally ruthless mistress, donna Violenta, is courteously kidnapped while bringing a message to the Spanish commander, don Diego Espinosa, which the rebels manage to substitute. She maintains an earlier pretense to be on Riccardo's side. When that fails, the tyrant couple kidnaps and tortures Laura.