The villainous Ormondo was in love with Isabella, who was happily married to Duke Bertrando. When she rejected his advances, Ormondo spread vicious rumours about her reputation and bribed Batone into casting her adrift in a boat on the sea. But she was rescued by a local miner, Tarabotto, who then disguised her as his niece, Nisa.
The opera begins ten years later: Duke Bertrando is due to visit the mines. Isabella, who is still in love with her husband, finally reveals her true identity to Tarabotto who promises to help her. The duke arrives with Ormondo and Batone. The duke is still in love with his wife even though he believes the rumours that she was unfaithful to him. Batone catches sight of "Nisa" and realises she is Isabella in disguise. He plots with Ormondo to abduct her that night but Tarabotto overhears their plan. The two villains are unmasked before they can kidnap Isabella, whose true story is revealed when she shows everyone her duchess' clothes and a portrait of the duke she has kept with her. Bertrando and Isabella are reunited.