Over the next few days, Percy settles into camp life. Annabeth teaches Percy Ancient Greek, and Percy finds it easier than English, which confirms his dyslexia works differently with Greek. Percy performs badly at most activities, especially archery, racing, and wrestling against Clarisse, and only really excels at canoeing. Because Percy does not clearly resemble any godly cabin, the older campers keep watching him and guessing about his divine parentage. Percy also keeps thinking about Sally and begins to resent how distant the gods are from their children.

During Percy’s first sword lesson, Luke trains cabin eleven in the arena and spars with Percy. Percy struggles with every practice blade, but after pouring water over his head during a break, Percy suddenly feels stronger and more coordinated. When Luke demonstrates a difficult disarming move, Percy unexpectedly turns the maneuver back on Luke and disarms him once, though he cannot repeat it. The moment makes Luke realize Percy may fight very well if he ever gets the right sword.

Later, Percy and Grover sit by the lake after training, and Grover admits Mr. D has not cleared his record yet. Grover explains that he can still earn his searcher’s license only if Percy receives a quest and Grover helps bring Percy back alive. Their talk turns to the empty cabins, and Grover explains the oath of the Big Three not to have more demigod children, the birth of Zeus’s daughter Thalia after that oath, and the monsters that hunted her until she died protecting her friends and became the pine tree guarding the camp. Percy asks whether anyone has ever returned someone from the Underworld, showing Percy is still thinking about rescuing Sally.

That night, camp plays capture the flag. Athena’s cabin allies with Apollo and Hermes against Ares and the remaining cabins, and Annabeth assigns Percy to guard the creek at the border. Percy soon realizes Annabeth has used him as bait, because Clarisse and several Ares campers attack Percy instead of defending their own side. Clarisse’s electric spear and the group’s assault overwhelm Percy until they wound him and shove him into the creek.

Once Percy lands in the water, everything changes. The creek restores Percy’s strength, sharpens Percy’s senses, and lets Percy defeat Clarisse’s group, even breaking Clarisse’s spear. While Percy holds off the attackers, Luke captures the enemy flag and wins the game for Percy’s side. Annabeth reveals she had planned the distraction, but then notices Percy’s sword wound healing unnaturally fast when Percy stands in the water. Percy steps out, weakens immediately, and confirms for Annabeth that the water is the source of Percy’s recovery.

Before they can process what this means, a hellhound bursts into camp and attacks Percy, proving someone inside the camp summoned it. Chiron kills the monster with a volley of arrows, and Annabeth orders Percy back into the creek, where Percy’s serious wounds heal before everyone’s eyes. As the campers watch, a glowing green trident appears above Percy’s head. Chiron declares Percy claimed at last: Percy is the son of Poseidon.