After Oepidus leaves Cornith to avoid killing his own father, he encounters and old man at the crossroads and accidentally kills him in a dispute over right of way. Oedipus continues along the road the Thebes, and answers the riddle of the Sphinx, free Thebes from her depredations. When he arrives at the city, he is acclaimed tyrant and is given the queen Jocasta’s hand in marriage. Thebes has been suffering from a plague. The Delphic oracle identifies the plague as ritual pollution caused by the death of Laius, the king. Oedipus rebukes the Thebans for not working harder to solve the mystery of Laius’ murder and end the plague.
As Thebans experience terrible suffering, Oedipus (even before the play begins) sends his brother-in-law, Creon, to ask the oracle of Delphi about the plague and how to relieve the city and citizens of their many problems. When he returns, Creon reports that the oracle said that they must find the murderer of the last king, Laius, and punish him accordingly in order for the plague to be lifted. Oedipus has a hard time believing that no one seemed to put forth much effort in attempting to find and punish the murderer before, and he criticizes the citizens for treating the murder of their leader in such a careless way. Oedipus is the king, and a respected one, and so no one is likely to protest much against his criticism, but the Chorus does state that there was a lot going on in Thebes at that time (i.e. the sphinx's rule had just ended, everything was in uproar after Laius's death, and so forth), and this made it difficult for them to do much.