The people of Thessaly have never warmed to the humbly born Griselda or forgiven their king, Gualtiero, for marrying her. To win them over he decides to test her publicly with an elaborate hoax – openly repudiating her and commanding her to leave the palace, abandoning her son. Fifteen years earlier he had pretended to kill their newborn daughter and engaged his faithful knight Corrado to spirit the baby away; this second incident gives the knight Ottone his long-awaited opportunity to declare his love for Griselda, who rejects it and remains loyal to King Gualtiero.

Gualtiero’s test includes sham plans to marry another woman, Costanza, who renounces her lover Roberto, prince of Athens, for the prospect of marrying the king ­ unaware that the king is her father and Griselda her mother. Only the king and Corrado, Roberto’s brother, know the truth of Costanza’s origins and secret upbringing.

When Gualtiero welcomes Costanza, apparently with romantic attentions, the jilted Roberto is distraught; facing exile, Griselda bids her son farewell when Ottone suddenly appears, wresting the boy from her embrace and kidnapping him.

Corrado questions Costanza about her feelings toward the king, whom she respects, and Roberto, whom she still loves. Dethroned and banished, Griselda is back in modest surroundings when she is confronted by Ottone, who is still possessed by his passions. He issues an ultimatum to Griselda: she must accept his love or see her son die. She wavers, but then resists his advances.

Wandering as she wrestles with conflicting allegiances, Costanza comes upon the sleeping Griselda and feels a strong bond with her. Griselda, once she awakens, believes she recognizes Costanza as her long-lost daughter. When Gualtiero appears, Corrado tells him that Ottone plans to abduct Griselda; the king, continuing his cruel charade, says he will leave Griselda to her fate.

Alone with Ottone, Griselda resists his continued advances. He tries to abduct her, but Gualtiero intervenes. Though he has Ottone arrested, Gualtiero rejects Griselda’s thanks as Costanza protests his cruelty.

As still another humiliation, Griselda has been returned to King Gualtiero’s household as a maid. But she remains faithful to him, and upon overhearing Costanza and Roberto renew their vows of love, she reproaches them severely for betraying the king. Gualtiero escalates his deception by insulting Griselda anew and commanding that the lovers, Costanza and Roberto, be true to each other ­ to Roberto’s delighted amazement. He further commands Griselda to accept Ottone or face execution. When Griselda chooses death rather than betray her honor, her nobility of spirit is finally unmistakable to everyone. Gualtiero announces he will restore her as is wife and queen, Ottone contritely begs forgiveness, Costanza regains her place as Griselda’s daughter and Roberto’s bride-to-be, and the people of Thessaly rejoice.