Percy begins to learn more about his world now that he knows the Greek gods are real. Grover explains that many of the great conflicts that have happened in the West have been due to the half-blood progeny of the gods. After World War II, Zeus, Poseidon, and Hades swore on the River Styx that they would not have children with mortals again because of this. So far, Zeus has slipped once, and his daughter Thalia died because of it. But the other Olympians have made no commitment and have not stopped having children, which is why Camp Half-Blood exists and why Keepers like Grover do their best to protect the half-blood children. Grover, who is charged with protecting Percy, has yet to succeed or fail in his task.

This week, the campers will play Capture the Flag, and Annabeth has teamed up with Cabin 11 against Clarisse and her allies. During his hero training, Percy has demonstrated some skill in swordsmanship and canoeing, but he is otherwise a mediocre addition to the team. Before the games begin, Annabeth tasks Percy with border patrol next to a lake before she and Luke set off in search of the flag. Clarisse and Ares’s other children gang up on Percy and begin attacking him ruthlessly. However, when Percy is backed into the lake, he feels a new sense of strength and awareness and defeats all of his attackers. Annabeth, who was wearing a Yankees cap to become invisible, reveals herself. Percy realizes that he was used as bait to distract Clarisse so Annabeth’s team could win the game. Annabeth explains that Athena “always, always has a plan.”

As Percy stands in the water before Chiron and the campers, a hellhound appears and attacks Percy. Chiron’s speedy archery saves Percy’s life. A glowing trident appears above Percy’s head. Percy is the son of Poseidon, the god of the sea. This complicates things: Poseidon is not supposed to have a son, and Zeus’s “master bolt” has recently been stolen. It is a weapon with the power to level mountains, and now that it has been revealed that Poseidon has a son, Zeus will consider Percy the most likely suspect for the theft. If Poseidon and Zeus go to war, the consequences could be catastrophic. Chiron proposes that Percy consult the Oracle and take on a quest. The Oracle delivers this prophecy:

You will go west, and face the god who has turned.
You shall find what was stolen, and see it safely returned.
You shall be betrayed by one who calls you a friend.
And you shall fail to save what matters most, in the end.

Afterward, Chiron shares that the real orchestrator of these events may well be Hades. The god of the underworld stands to gain a great deal if Zeus and Poseidon fight each other—and both Zeus and Poseidon have broken the oath they swore on the River Styx. Consequently, Chiron proposes that Percy travel over land to Los Angeles, the gateway to Hades in the modern world. There, he will have to fight to retrieve the master bolt for Zeus. It is a dangerous quest. However, it could avert war. Grover and Annabeth will accompany him.

Percy accepts the quest. Before he leaves, Luke gives him a present: shoes that fly on command. However, because the sky belongs to Zeus, Percy gives the shoes to Grover. Chiron, meanwhile, gives Percy a sword—one that can be disguised as a pen. The sword is named “Riptide,” and it is made of celestial bronze. It will not hurt mortals, but it is a powerful weapon against mythological monsters. The trio of heroes hardly makes it out of the camp before Percy is given a chance to defend himself. The Furies board Percy’s bus and attack. The young heroes barely manage to escape into the woods.