Medea is set in the ancient Greek city-state of Corinth. Jason, the heroic son of King Aeson of Iolcus, has left his wife, Medea, and married the princess of Corinth. As the play begins, the Nurse, Medea's slave, gives a monologue summarizing events that took place before the play began. Jason had been given the task of capturing the Golden Fleece by the king, Pelias, who took the throne of Iolcus away from Jason's father. The Golden Fleece, a ram's gold skin, is defended by a dragon in Colchis, a region on the Black Sea. With a group of men called the Argonauts, Jason sailed to Colchis in the Argo and enlisted the help of Medea, the king's daughter, to carry out the task. Medea, who has magical powers, fell passionately in love with Jason. She not only helped him, betraying her own family, but married him. She then conspired to murder Pelias through trickery, which forced the couple into exile in Corinth. They have two sons, but Jason wants more wealth and so has left Medea for his new bride, the daughter of King Creon of Corinth.
Medea is mad with rage at being dishonored and abandoned. The Nurse hears her crying to the gods from within her house and worries about what Medea will do in her dangerous state of mind. The Chorus—a group of Corinthian women who are Medea's friends and serve as the voice of Greek society in the play—arrives onstage, and the Nurse fetches Medea to speak to the women. Medea, however, will not be consoled. A divorced woman has no respect, she tells them; she has no city, no protection, and no relatives to help her.
King Creon arrives to order Medea and the children into exile, because he fears Medea will harm his daughter, given her experience in "evil ways." After Medea begs to remain for one day, the king grants her wish—foolishly, for Medea begins plotting the murder of his daughter. Jason appears to say that Medea deserves her exile for slandering the royal house. When Medea reminds him of all the crimes she committed to help him and of their children, Jason belittles her help. He claims he did more for her than she for him and says he's marrying the princess to give his children financial security. Medea refuses his offer of help, saying,"Gifts from a worthless man are without value."
When Aegeus, the king of Athens, comes to ask Medea for some advice, Medea asks him to take her in, and he agrees. After he exits Medea reveals to the Chorus her plan to send the children to the princess with a poisoned robe and tiara, then kill the children. She feels she has no other choice with "no father, no home, no refuge." Soon a messenger from Creon's house comes to say the princess and king are both dead; in trying to lift his dead daughter, the king became entangled in the poisoned robe and died. Medea next enters the house to kill the children, and the audience hears their cries for help.
Jason arrives to the news that the boys are dead. As Medea rises above the house in a winged chariot, the bodies of the children inside, she taunts Jason: she has finally moved his heart. She flies off to "Hera's sacred lands/in Acraia" to bury her children and then go to Athens. The Chorus comments, "What we don't expect/some god finds a way/to make it happen."