Oedipus Rex by Sophocles opens with Oedipus ruling the city of Thebes. A plague has descended upon the city. The cause of the plague is the anger of the gods, but Oedipus, his advisers, and the citizens of Thebes are uncertain what they have done to bring the gods' anger upon them. The plot of the play revolves around the efforts of Oedipus and the other residents of the city to uncover the reasons why the gods cursed them with the plague.
Eventually, through the medium of the prophet Teiresias and inquiries into the past history of Thebes, we discover the reason for the ritual pollution which has caused the plague. We discover that Oedipus accidentally killed his own father Laius and married his mother Jocasta. These events are in part due to a curse that Laius brought down upon the family.