Creonte and his daughter Glauce await her upcoming wedding to Giasone. Her joy is overshadowed by fear of Medea, Giasone’s spurned wife and mother of his two sons. As her friends try to reassure her, she prays for a blessing on her marriage to Giasone. When Giasone arrives in Corinth with his two young sons, Creonte swears to protect both Giasone and his children. Giasone tenderly comforts Glauce, but her fears turn to reality when Medea appears and denounces their pending marriage.
Creonte banishes Medea, but allows her one final meeting with Giasone. She appeals to him, as the mother of their children, to return to her. But Giasone is determined to marry Glauce, and Medea curses him, swearing revenge. The abandoned and furious Medea calls for bloodshed and terror for Creonte and his daughter Glauce.
Medea’s confidant, Neris, warns her that Creonte wants her to leave the city immediately. Medea confronts Creonte, begging him for one more day to say goodbye to her children. He reluctantly agrees. As Neris sings of the sad situation, Medea considers Giasone’s affection for their children and plots her unthinkable revenge. Medea, appearing to have calmed, sends the two boys to Glauce with wedding gifts that have been doused in poison. As the wedding of Giasone and Glauce begins, Medea calls on the god of marriage to bless her murderous plot.
Neris leads the children to present Medea’s gifts to Glauce. Left alone, Medea reveals that her plot for vengeance against Giasone is to murder their children. Neris reappears with the children for their last visit with Medea. She tells Medea that Glauce has accepted her gifts. Medea can barely look at her sons, and tells Neris that her revenge against Giasone is not yet complete. Neris begs her not to harm the children, and Medea appears to relent.
A commotion erupts, with crowds lamenting the death of Glauce, poisoned by one of Medea’s gifts. The grief-stricken Giasone emerges, along with an angry mob, searching for his children who have been led away by Medea. As the crowd gathers and Giasone prays for the safety of his sons, Medea reappears. Giasone desperately asks her, “where are my children?” Medea replies, “They were your children.”