Greek tragedies were based on widely-known myths or famous historical events, so the audience would know the characters and outline of the story they were about to see. Seeing a play about Oedipus, for instance, Sophocles’ Athenian audience would already know that this story came from the cycle of myths about the city of Thebes, one of Athens’ rivals in the 5th century. Most surprises did not come from the plot, but from the new way the playwright used familiar material.
The Oedipus story is set a few generations before the Trojan War, which the ancient Greeks placed in 1184 BCE. King Laius of Thebes received a prophecy that his son would kill him. To avoid the outcome of the prophecy, Laius had his baby exposed (abandoned without protection from the elements—a common way to get rid of unwanted infants) on Mount Cithaeron, one of the most remote points of his kingdom. As an extra precaution, he nailed the child’s feet together. Unfortunately for Laius, the baby survived and was raised as a prince of the city of Corinth. He was named Oedipus, which means “swollen feet” in Greek.
Many years later, Oedipus, not knowing his true birth, met Laius on the road and killed him. At the time, Thebes was being terrorized by a monster with the head of a woman, body of a lion, and wings of an eagle called the Sphinx. She was particularly famous for telling everyone she encountered a riddle: “What walks on four legs in the morning, two legs in the afternoon, and three legs in the evening?” Men who answered incorrectly were devoured.
Oedipus answered the challenge by guessing “man” (who crawls as a baby, walks on two legs as an adult, and leans on a cane in his old age). Her riddle solved, the Sphinx threw herself from a cliff, and Oedipus was crowned king of Thebes. Oedipus married the recently widowed queen, Jocasta. He did not know his real relationship to the man he killed and the woman he married.
Because Sophocles’ audience was already familiar with this information, he does not need to explain it in the drama; he can simply allude to it.