Years before the action begins, Gualtiero, King of Sicily, had married a poor shepherdess, Griselda. The marriage was deeply unpopular with the king's subjects and when a daughter, Costanza, was born, the king had to pretend to have her killed while secretly sending her to be brought up by Prince Corrado of Apulia. Now, faced with another rebellion from the Sicilians, Gualtiero is forced to renounce Griselda and promises to take a new wife. The proposed bride is in fact Costanza, who is unaware of her true parentage. She is in love with Corrado's younger brother, Roberto, and the thought of being forced to marry Gualtiero drives her to despair.
Griselda returns to her home in the countryside where she is pursued by the courtier Ottone, who is in love with her. She angrily rejects his advances. Gualtiero and his followers go out hunting and come across Griselda's cottage. Gualtiero foils an attempt by Ottone to kidnap Griselda and allows her back to the court, but only as Costanza's slave.
Ottone still resolutely pursues Griselda and Gualtiero promises him her hand as soon as he himself has married Costanza. Griselda declares she would rather die and, moved by her faithfulness, Gualtiero takes her back as his wife. He reveals the true identity of Costanza and allows her to marry Roberto.