Percy returns to Camp Half-Blood and immediately feels that it has changed for the worse. The grounds look familiar, but the camp now feels militarized and sick, with weapons being stockpiled, nervous magical beings on alert, and the forest and fields showing signs of the poisoned borders. Tyson is amazed by everything he sees, while Percy reflects on his own isolation in Poseidon’s cabin and the dangerous history of children of the Big Three.

At the Big House, Percy, Annabeth, and Tyson find Chiron packing to leave. Chiron confirms that he has been blamed and dismissed after Thalia’s tree was poisoned by a deadly substance from the Underworld, likely tied to Kronos’s forces, though Chiron refuses to say more openly. He explains that the poison is killing the tree and weakening the camp’s magical defenses, and that only a lost source of magic might save it. Chiron warns Percy not to leave camp or rush into action, fearing this crisis may be a trap, and he makes Annabeth swear on the River Styx that she will keep Percy safe.

After Chiron departs for exile in the Everglades, Percy, Annabeth, and Tyson go to dinner. Percy notices the camp hierarchy settling back into place, including Annabeth leading Athena’s cabin and Clarisse, still injured, leading Ares. At the head table, Percy meets Tantalus, the new activities director, a bitter spirit from the Fields of Punishment who is cursed to be forever unable to eat or drink. Tantalus and Dionysus mock Percy, blame him for trouble, and insist that Tyson be kept apart while they decide what to do with him.

Percy makes an offering to Poseidon but remains angry and helpless. During announcements, Tantalus reveals his priorities by reinstating the dangerous chariot races, exciting many campers even though camp security is failing. When Clarisse raises concern about patrol duty and the poisoned tree, Tantalus dismisses her. He then publicly treats Tyson as a monster, asking the camp where he should be confined and suggesting he is fit only for menial labor.

Before Tantalus can decide Tyson’s fate, a glowing green trident appears above Tyson’s head. The symbol claims Tyson as a son of Poseidon, just as Percy was claimed before. Instead of responding with reverence, most of the camp laughs at the implication that Percy and Tyson are related. Percy realizes that Tyson is not just a Cyclops but his half-brother, and that Tyson will now share Poseidon’s cabin with him.