What is happening in Thebes as the beginning of the play?

At the beginning of the play Oedipus Rex, Thebes is being devastated by a city-wide plague. Many people are falling ill, and Oedipus begins to believe that there is a divine cause for the plague. He sends his men to the Oracle at Delphi to determine the will of the Gods and see why they are having such a destructive plague.
Little does he know that he is the reason behind the plague that is occurring. After inquiring the will of the Gods, he learns that they are punishing the city because the murderer of the previous king was never brought to justice. Oedipus vows to bring the murderer to bear for his crimes, but Tiberias warns against making strong claims and vowing punishment on the individual, because the prophet knows that Oedipus is the truly guilty party.