Jocasta enters and makes an offering to Apollo to appease Oedipus's mind. Just then, a messenger—an old man—arrives from Corinth, with news that the people there want to make Oedipus their king. Polybus, king of Corinth—the man Oedipus believes to be his father—has died. Jocasta is overjoyed because she views Polybus's death as further proof that the prophecies are false.
The news from Corinth seems like further evidence to support Jocasta's claim that prophecies are meaningless. If King Polybus has died of natural causes, then Oedipus can't fulfill the prophecy and kill his own father.
Oedipus enters and learns the news. Relieved, he celebrates with Jocasta and agrees with her that the oracles and prophecies are "dead," and that chance alone rules the world.
The idea that chance, rather than the gods, rules the world is deeply blasphemous. It is significant that from this moment on, things come crashing down.
Jocasta urges Oedipus to live without fear. Yet Oedipus admits that because his mother is still alive, part of the prophecy might still come true.
Even so, Oedipus is not completely able to deny either his guilt or his belief in fate.
The messenger asks what Oedipus is afraid of. Oedipus tells him the prophecy—that he would kill his father and sleep with his mother—and says that this is why he has never returned to Corinth. The messenger tells Oedipus he never had anything to fear. Polybus and Merope weren't his real father and mother.
By leaving Corinth, Oedipus thought he was thwarting the prophecy, but instead he was carrying it out. Here the messenger thinks he is helping Oedipus, but is in fact dooming him. Fate is unavoidable.
The messenger tells Oedipus that he (the messenger) came upon a baby on the side of Mount Cithaeron, near Thebes. He freed the baby's ankles, which were pinned together, and gave the baby to Polybus to raise as a gift. That baby grew up to be Oedipus, who still walks with a limp because of the injury to his ankles. When Oedipus asks for more details about who his parents were, the messenger says he doesn't know, but was given the baby by another shepherd who was a servant of Laius.
The detail about the pinned ankles links Oedipus to the baby who Jocasta and Laius tried to kill. Oedipus's swollen ankles are marks of his fate. Yet Oedipus, who solved the riddle of the Sphinx, still can't see it. His search for the truth has actually blinded him to the truth.
Jocasta reacts sharply to this last piece of news. Meanwhile, the chorus tells Oedipus that this other shepherd, Laius's old servant, is the same man as the eyewitness to the murder of Laius.
Jocasta has realized the awful truth: her current husband is in fact her son.
Jocasta now begs Oedipus to abandon his search for his origins. Oedipus thinks she's worried that he will discover he's the son of some slave or commoner, a fact that might shame her. She insists that isn't it, and continues to beg him not to question the shepherd. He won't listen to her. At last, she lets out a wrenching scream, calls Oedipus a "man of agony," and flees through the palace.
Though she knows the truth, Jocasta desperately wants to hide from it, hoping that by maintaining some bit of doubt they might escape their guilt and fate. But Oedipus is still blind, and refuses to stop. He relentlessly pursues the very truth that will destroy him.
Oedipus declares that he must know the secret of his birth, no matter how common his origins. A shepherd approaches. The messenger confirms that it's the same man who gave him the baby. Oedipus and the messenger question the old shepherd. When they bring up the subject of the baby, the shepherd refuses to speak.
This is a moment of great dramatic irony, when the audience knows the truth, and other characters know the truth, but the main character still does not. As many characters have before, the shepherd tries to stop the discovery of the truth.
Only after Oedipus threatens to torture the shepherd does the shepherd admit that he gave the baby to the messenger. The shepherd then refuses to name the father and mother of the baby. Oedipus threatens to kill the shepherd if he does not speak. Finally, the shepherd gives in: the parents of the baby were Laius and Jocasta. The shepherd says he was told to kill the baby boy because of a prophecy that he would grow up to kill his father. But the shepherd took pity on the baby and gave it to the messenger.
The shepherd is the last roadblock between Oedipus and disaster, and fittingly, he is the most reluctant to speak. In his blind need to know the truth, Oedipus forces his way past every obstacle. He truly dooms himself, even going so far as to threaten to kill the shepherd, to make him speak the very words which seal Oedipus's fate.
Realizing who he is, and that the prophecies have come to pass, Oedipus lets out a terrible cry and rushes into the palace. The messenger and shepherd exit.
Now Oedipus knows everything. His fate is revealed, his blindness lifted, and his guilt and shame descend upon him.