This summary will try to address both the version of the Medea staged by Euripides in 431 BCE and the play written by Seneca in the middle of the first century CE.
The play begins with the news that Jason will divorce Medea and marry the daughter of Creon, who is the king of Corinth. Creon tries to banish Medea from Corinth, but Medea succeeds in managing to get Creon to allow her to stay for one more day in Corinth.
In Euripides' version of the play, Medea encounters Aegeus, the king of Athens, who is passing through Corinth. Aegeus agrees to grant Medea asylum in exchange for her help with his childlessness. As a "witch," Medea knows all sorts of magic potions that can remedy Aegeus' childlessness. Seneca omits the scene with Aegeus.
Of course, Medea is furious with Jason for divorcing her, but she pretends to make up with Jason so that she can gain access to Creon's daughter to destroy her. Medea creates some poisoned garments and uses her own children to deliver these garments to the princess.
When Creon's daughter puts on the poisoned clothing, she is killed, as is her father Creon, who also becomes stuck in the deadly garment.
Before Medea flees from Corinth, she also kills the children that she has had by Jason. The play ends with Jason left to live out what is left of his miserable life.