The Athenian audience who saw the first performance of Euripides' Medea at the state dramatic contest in 431 BC and who awarded the third prize to Euripides would have been familiar with the whole story of the chief characters, and we, twenty-three centuries later, are handicapped in our understanding of the play if we have not at least some knowledge of the same story.
The Athenians would have known Medea as a barbarian princess and as a sorceress, related to the gods. She came from the faraway land of Colchis at the eastern extremity of the Black Sea, where her father, King Aeetes, a sorceror himself and the son of Helius, god of the sun, kept the Golden Fleece. Here Jason had come with the Argonauts, the first expedition of western Greeks against the eastern barbarians. Medea had fallen in love with him, and by her aid he was able to avoid the traps laid for him by Aeetes, to regain the Golden Fleece, and to escape, taking Medea with him. She, to assist the escape, had murdered her own brother, strewing the pieces of his body over the water so that her father's fleet, while collecting the fragments for burial, might lose time in the pursuit of the fugitives.
Medea and Jason then settled in Jason's hereditary kingdom of Iolcus, where Pelias, his uncle, still cheated him of his rights. Medea, hoping to do Jason a favor, persuaded the daughters of Pelias to attempt, under her guidance, a magic rejuvenation of their father. The old man was to be killed, cut in pieces, and then, with the aid of herbs and incantations, restored to his youth. The unsuspecting daugthers did as they were told, and Medea left them with their father's blood upon their hands. However, the result of this crime was no advancement for Jason but rather exile for him, Medea, and their two children.
From Iolcus they came to Corinth, the scene of Euripides' play. Here Jason, either, as he says himself, wishing to strengthen his own economic position, or, as Medea thinks, because he was tired of his dangerous foreign wife, put her aside and married the daughter of Creon, king of Corinth. It is at this point that the action of the play begins; but the Athenian audience would know well enough what the plot would be. They would know that Medea, in her jealous rage, would destroy both Creon and his daughter by means of a poisoned robe which clung to the flesh and burned it; that, despairing of her children's safety and wishing through them to injure Jason in every way, she would kill them with her own hands; and that, finally, by supernatural means, she would escape to their own city and take refuge with the old King Aegeus.

Medea, princess of Colchis and wife of
Jason, son of Aeson, king of Iolcus
Two children of Medea and Jason
Creon, king of Corinth
Aegeus, king of Athens
Nurse to Medea
Tutor to Medea's children
Messenger
Chorus of Corinthian Women