Dircé, daughter of Creon, king of Corinth, is to marry Jason, the hero who has recovered the Golden Fleece, but she is worried about the revenge of Medea, who is known to indulge in the occult arts. The latter has two children from Jason who repudiated her. Créon reassures Jason: he will protect his two children so that they do not pay for their mother's faults.
The Argonauts come to present their homage to the bride and spread at her feet the Golden Fleece taken away in Colchis. Dircé is troubled again when she hears the name of Medea's country. Jason tries to reassure her when a guard announces the entrance of a veiled woman: Medea who wants Jason to come back to her. Faced with his refusal, the magician threatens revenge. Left alone with Jason, she reminds him in vain of the happy days spent together and threatens him: “You will never see your wedding day”
Néris, servant of Medea, tries in vain to console the latter and convince her to leave Corinth to escape the wrath of the people.
Creon and his escort order Medea to leave the city. She accepts and asks permission to say goodbye to her children, which the king grants her. Neris consoles Medea, but the sorceress has hatched a plan: her children will be the instruments of her revenge and the grief of the traitor Jason will be boundless.
When the hero arrives, Medea pretends to be very sad about the separation from her children. She orders Néris to give Dircé her wedding gift: the tiara and mantle given to her by Apollo, but she still plans revenge. When the ceremony is over, she grabs a candle from the altar and leaves.
Somewhere in the mountains, Medea prays to the Gods to give her the courage to carry out her terrible revenge. Néris, after giving Dircé the magician's gift, returns to the two children. Medea brandishes her dagger to kill them, but is unable to accomplish her gesture. She then reveals her ruse to Néris: the diadem and the mantle are poisoned and Dircé will die of it. Horrified, Neris flees to the temple of Juno with the children. As the magician had planned, Dircé dies and Jason, in despair, wants to stop Medea. She runs to take refuge in the temple and comes out flanked by the three Furies brandishing the bloody dagger with which she killed her children. She sets fire to the temple and, as she returns inside, announces to the unfortunate Jason: “I am going to the sacred river! My shadow will be waiting for you there!”