The three unities refer to action, time and place: The action should be one whole and take place over one day and in one place.
In this play, Medea's husband, Jason, has abandoned her and their children for another woman. Afraid that she might harm the children in her jealous rage, her father has banished her from Corinth. She has begged for and been granted just one more day to prepare. There, in a nutshell, is how Euripides incorporate the unities into this play. The action is Medea's revenge against Jason. It takes place in one city, Corinth, over the course of one day.
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