Get Oedipus Rex straight once, then move.

The play opens in Thebes, which is suffering from a plague. Citizens beg Oedipus to help them. He has already sent his brother-in-law Creon to the oracle at Delphi, and Creon returns with the god's answer: the plague will end when the murderer of the former king, Laius, is found and punished.

Oedipus throws himself into the investigation. He publicly curses the killer, not knowing he is cursing himself. He summons the blind prophet Tiresias, who refuses to speak at first, then finally tells Oedipus that Oedipus himself is the murderer. Oedipus dismisses this as a political conspiracy between Tiresias and Creon. He accuses Creon of trying to take the throne.

Jocasta, Oedipus's wife and queen, tries to calm him. She tells him prophecies are worthless, pointing out that an oracle once told her and Laius their son would kill Laius, but they had the baby's feet pierced and left him on a hillside to die. Laius was later killed by strangers at a crossroads, she says, not by any son. This detail about the crossroads makes Oedipus go cold. He begins to remember a violent encounter he had at a crossroads years ago, before he came to Thebes.

Oedipus tells Jocasta his history. He grew up in Corinth as the son of King Polybus and Queen Merope. A drunk man once told him he was not really their son. He went to the oracle at Delphi, which told him he would kill his father and marry his mother. To escape that fate, he left Corinth and never went back. On the road, he got into a fight with a group of travelers and killed them. He then arrived in Thebes, solved the riddle of the Sphinx, and was given the throne and the widowed queen as a reward.

A messenger arrives from Corinth to report that Polybus has died of old age. Oedipus is briefly relieved, thinking the prophecy is broken, but then worries he might still marry his mother. The messenger, trying to comfort him, reveals that Oedipus was not actually Polybus's biological son. He was a foundling, given to the Corinthian messenger as a baby by a Theban shepherd.

Jocasta understands the full truth at this point and begs Oedipus to stop asking questions. He refuses. The shepherd is brought in and, under pressure, confirms everything: the baby was Laius and Jocasta's son, given to the shepherd to be left to die, but instead passed on to Corinth. Oedipus is that baby. He killed Laius at the crossroads. He married his own mother.