The first half sums up the story of Jason and the Argonauts as they travel to Medea's barbarian land in search of the golden fleece. In fitting with the soundtrack (which features North African Tribal music), Pasolini depicts Medea's people as a tribal people who perform the ancient seasonal rituals and sacrifices to secure their harvests. Their costumes and dances are based on those of Eastern European Mummers such as the Romanian Calusari ceremonies and their counterparts in the Balkans.
A young man is offered up as a gruesome human sacrifice and his organs and blood are sprinkled over the crops in a ritual sparagmos. The depiction of the sacrifice, in which the victim is bound to a wooden structure and the villagers not only fertilize crops with his body and blood but also consume them, has obvious parallels to the Christian rite of communion. This shows the influence of anthropological research and the theme of the sacrificed king and the Dying God that Sir James George Frazer and the Cambridge Ritualists popularised, but which has been well documented by other researchers, ancient historians and anthropologists. The young man has been offered up is a substitute for her brother Absyrtus, who is whipped by the tribal priests at the same time so that his suffering parallels that of the chosen royal scapegoat.
Meanwhile, Jason and his comrades have arrived and are shown pillaging the locals. Medea awakens her brother Absyrtus to help her steal the Golden Fleece, which they then deliver to Jason. The Argonauts hastily depart from Colchis, with Medea's father in hot pursuit. When the Colchians begin closing in, Medea kills her brother and dismembers his body, in a manner similar to the sacrificial victim of the sparagmos ritual earlier. Her father's men are then forced to halt and retrieve the scattered pieces of his son's body, enabling Jason and Medea to escape.
When they return to Jason's homeland as husband and wife, Medea is stripped of her ornate barbarian garb and dressed in the garments of a traditional Greek housewife. The film generally follows the plot of the play by Euripides from this point onward, though it takes some liberties with the chronology of events and deepens the human motivation.
Jason has two sons by Medea, but later decides to opt out of his union with her in favor of a more lucrative and culturally acceptable marriage to the Corinthian princess Glauce. Enraged at his betrayal and the disinheritance of his children, Medea plots revenge against Jason and his new bride and sends Glauce a robe bewitched with magic herbs.
Here Pasolini introduced an important element of his own. There are two versions of the destruction of Glauce and her father in the film. The first follows the traditional legend, when the princess puts on the robe, the garment catches fire and burns her alive, along with her father Creon, who attempts to douse the flames.
The second version is more powerful and shows us a Glauce who, like Medea, has become a pawn in a patriarchal society. The robe she puts on is in fact that which Medea wore when she served as High Priestess. Glauce looking in the mirror begins to realise what Medea has lost and out of shared grief leaps to her death, followed by her father who is helpless to save her.
Medea then proceeded to kill her own sons by Jason and sets fire to their house, and as a final injury refuses to give Jason the bodies of their children for burial.