Final answer:
Tiresias's prophecy is crucial to the plot, driving Oedipus's actions and leading to the tragic realization and fulfillment of the prophecy.
Step-by-step explanation:
Tiresias's prophecy significantly affects the plot of the story by setting Oedipus on the distressing journey of self-discovery. Tiresias predicts that Oedipus will end up killing his father and marrying his mother, a prophecy Oedipus tries desperately to avoid, which ironically leads him directly into its fulfillment. The prophecy acts as a catalyst for the main events in the play, fueling Oedipus's quest for truth and leading to the eventual unraveling of his identity, and his downfall.