Gualtiero; king of Thessaly, has married Griselda, a simple shepherdess whom the people have never forgiven for being of lowly origin. In order to impose the queen on his subjects, he decides to put her publicly to the test by repudiating her and asking for the hand in marriage of Princess Costanza. The latter, unknown to all except Gualtiero and the faithful Corrado, is none other than the royal couple's daughter, whom her father spirited away at birth from the public condemnation of his subjects and subsequently had brought up in secret.

Act I

On the occasion of the solemn audience given each year before the people, Gualtiero announces his decision to repudiate Griselda and commands her to leave the palace, abandoning her son. The knight Ottone, long enamoured of the queen, seizes this opportunity to declare his love. He is repulsed by Griselda, who remains faithful to the king. Costanza, unaware that the man she is supposed to be marrying is in fact her father, prepares to take leave of her lover Roberto, prince of Athens.

Gualtiero welcomes his betrothed with words of affection which attract Costanza but drive Roberto to despair. As she is about to leave the palace, Griselda embraces her son for the last time. Ottone suddenly appears, tears the child from his mother's arms and escapes. Corrado swears to Griselda that he will do everything possible to find the boy.

Act II

Costanza, questioned by Corrado about her feelings towards the king, is reminded of her former vows. Enter Roberto, whom she is forced to reject. Griselda, dressed as a shepherdess, has withdrawn to a hut in the woods. Ottone joins her there and renews his advances. Enjoined to choose between marrying the knight and seeing her son die, the queen wavers but resists.

Costanza, wandering in the forest, encounters Roberto, then comes upon Griselda. The latter believes she recognises her lost daughter in the princess. Enter Gualtiero, to whom Corrado announces that Ottone is preparing to abduct Griselda. Reaching the height of his feigned cruelty, the king decides to abandon her to her fate. Facing Ottone alone, Griselda once more resists his advances. When he attempts to abduct her, Gualtiero, moved by Costanza's pleas, intervenes to stop him. Ottone is arrested. When Griselda thanks Gualtiero's emotional protests.

Act III

Costanza and Roberto swear to be faithful to one another, in union or in death. Griselda overhears their oath and heaps reproaches on them. Gualtiero learns of Costanza's betrayal from Corrado, but to general astonishment, turns on Griselda, whom he outrages once more before commanding the lovers to be faithful to their flame. Roberto gives voice to his reviving hopes, while Costanza is dumbfounded by this sudden reversal of fortune.

Gualtiero announces his decision to offer Griselda to Ottone. Now that the queen's torment is at its peak, Gualtiero falters, incapable of assuming his cruel double game any longer. But he regains control of himself and publicly offers Griselda to Ottone, threatening her with death if she refuses. The queen once again resists.

Now all bow before so noble a spirit, and Gualtiero solemnly declares his intention of taking his beloved wife back to his side. Ottone beseeches his masters' forgiveness. Costanza is restored to her mother and her lover, and all Thessaly sings the joys of love and the virtue of the royal couple.