Oedipus' downfall begins as soon as he is left for dead by his father as an infant in a response to the prophecy that Oedipus would murder his father and marry his mother. Oedipus unknowingly goes on to murder his father and marry his mother, proving his fate was predetermined.
Oedipus travels to Delphi to speak with the Oracle in an attempt to learn the truth about his parents. The Oracle simply tells him that he is going to murder his father and have sexual relations with his mother. Oedipus, thinking his parents live in a different town, goes to Thebes in an attempt to stave off the prophecy. Along the way he runs into his true father, King Laius, and murders him after an altercation between the two. Oedipus arrives in Thebes and outwits the Sphinx, which saves the city. In his honor, the citizens make him their king and implore him to marry the king's widow, Jocasta. This seals his fate, as Oedipus has now killed his father and married his mother. Once he learns of the truth, he blinds himself and goes into exile.
The moral behind the story is that a man cannot run or change his fate once it has been determined. The character of Oedipus was part of "The Theban Plays," a trilogy written by Sophocles. The philosopher Aristotle claimed the play "Oedipus The King," which is included in the trilogy, was the greatest example of tragedy.