The King Must Die is the story of the hero Theseus. The tale begins in Troizen, the land of Theseus' grandfather, King Pittheus. Theseus believes that he is the son of the sea-god Poseidon. At an early age he discovers that he can sense earthquakes. He interprets this skill as proof of his heritage. Small for his age, Theseus learns to outwrestle larger youths by using his agility. At seventeen he finds out that his father is Aigeus, King of Athens, although Pittheus tells him that Poseidon may have had something to do with his birth. Theseus decides to go to Athens, and he does not take a Cretan ship there after the Cretans in the harbor insult some of his people. Instead he goes through the dangerous Isthmus Road with Dexios, his friend, as charioteer. Dexios is murdered on the journey, but Theseus avenges him and arrives unharmed.

Theseus comes to Eleusis on the day when the king must die, and the queen tells Theseus that he must wrestle with the current king. In Eleusis, as in some other kingdoms of the Minyans, Mother Dia is the supreme deity, and the queen reigns supreme. The king, named Kerkyon in Eleusis, is killed each year. After Theseus kills the king, he sleeps with the queen and becomes Kerkyon himself. But Theseus does not come from the same background as the other people from Eleusis, and he does not act the same as other Kerkyons before him. He trains his personal guard, the Companions, and makes them warriors. He proves to the queen that he is in charge. Theseus befriends Pylas, the heir to the neighboring Megarian throne, and together the two kingdoms wipe out all of the thieves in the Isthmus Road. Xanthos, the Queen's brother, tries to have Theseus killed in the battle, but Theseus kills him in armed combat. Then Theseus goes on to Athens to meet his father.

He arrives in Athens, and greets the king. Just before Theseus tells his father who he really is, his father realizes that Theseus is his son and knocks away a cup of poisoned wine that the Lady Medea wants him to give to Theseus. Theseus learns that Aigeus intended to poison the King of Eleusis in order to lift a curse from Mother Dia, but when he sees that he is supposed to kill his own son, he refuses. Medea curses Theseus and then flees, and she is never found. Aigeus needs to fight a war to keep his kingdom, but Theseus tells his father he must first return to Eleusis, and he does so against his father's wishes. When he gets to Eleusis he meets the Companions, who have heard that he traded Eleusis to Athens in return for being pronounced heir to the Athenian throne. He reveals his heritage, makes a rousing speech to the army, and returns home triumphant. He learns that the Queen is dying, having poisoned herself with the House Snake. Theseus changes some of the customs in Eleusis, and helps his father win a war in Athens.

After his father tries to protect him from the Cretan lottery, Theseus volunteers to become a bull-dancer. Along with seven girls and six boys, all from Athens or Eleusis, Theseus heads to Crete as a slave. In Crete he meets Asterion. Minos' heir is evil and hungry for power. Although his attempt to embarrass Theseus fails, he still treats the King of Eleusis as an inferior. They go to Knossos Palace, known as the House of the Ax, a place whose immensity shocks Theseus. There, the group of slaves, bound together by an oath and calling themselves the Cranes, begins training to be bull-dancers. Theseus watches the Corinthian, who is the best of the bull-leapers, the most prized position for a bull-dancer. Over time, the Cranes become the best group and Theseus becomes the best bull-leaper. Ariadne, Minos' daughter and the Goddess-on-Earth, falls in love with him, and he begins plans to escape from Crete. He meets King Minos, who has leprosy, and learns that the King wants him to marry Ariadne and become the next Minos. But Asterion is very powerful and it will be tough to oppose him. Minos has Theseus sacrifice him with the sacred ax Labrys.

The next day, Asterion tries to fix the bull-dance and have Theseus killed. Although Theseus survives, everyone expects Poseidon to be angered, because his sacred rite has been tampered with. The following morning there are two small earthquakes, and Theseus realizes there is going to be a great earthquake. The bull-dancers break out of the Bull Court, and get outside of the Palace, and Theseus goes back for Ariadne just as the quake hits. He saves her, and then Theseus leads a charge into the remnants of the Palace to kill Asterion, with the native Cretans supporting them. He finds Asterion in the midst of being anointed the next Minos and kills him. They leave the island to return to their homes, bringing Ariadne with them. They stop on the island of Naxos, where Ariadne is highly respected and the Queen there invites them to stay for the feast of Dionysos the next day. At the feast, Ariadne takes part in the sacrifice of the King, and her role is so horrifying to Theseus that he leaves the island without her. He returns to Athens, and learns that his father has committed suicide. Years later, after many other adventures and hardship, in his old age, when Poseidon no longer speaks to him, Theseus knows only that the fate of man is beyond his ability to understand.