Thebes was stuck with the sphinx and couldn't solve its riddle. Till Oedipus came and was able to solve the riddle defeating the sphinx.
In return for Oedipus saving them they made Oedipus king by getting him to marry their queen Jocasta.
Later on the priest of Thebes asks Oedipus if he can save Thebes once more. He tells him that Thebes is in danger that there is blight in their fruits, their cattle is dying, and women are giving birth to dead children. So Oedipus sends Creon the brother of Jocasta to Apollos oracle to see what is going on in the city
Creon finally returns and tells Oedipus that we have to exile or kill the murderer of Laius who was the king before hand. Oedipus asks if we have any clues or idea who it may be. Creon says that all he knows is that he is in Thebes and we must exile or murder the killer to save Thebes.
Oedipus calls the prophet to come but when he arrives he says he wish to not speak who the murderer is and wants to go home. This made Oedipus suspicious of the prophet and accuses him of helping the killer kill Laius which then the prophet says that Oedipus was the one who killed him. Later Oedipus accuses both Creon and the prophet for being the killers and trying to overthrow him which Creon rejects saying he doesn't want such responsibility.
Later on the queen enters the room wondering what the commotion is all about and also says that her brother is not the murderer since she knew how Laius died. She then explains how Laius died which made Oedipus worry that he may have killed him since a similar incident happen to him when he was on his way to Thebes and tells his story to Jocasta which also made the queen tell the story about her son and the prophecy that would happen if the son lived his life he would have killed his father. Now Oedipus waits for a survivor to tell him if there were multiple or one who killed Laius and his men to see if Oedipus is the killer or not