In the days before the chariot race, Percy is miserable after Tyson moves into the Poseidon cabin as his newly claimed half-brother. Other campers mock Percy for being related to a Cyclops, and Percy feels ashamed, angry at Poseidon, and unsure how to treat Tyson. When Annabeth tries to help Percy focus on building a race chariot together, the conversation turns into an argument because Annabeth openly distrusts Cyclopes and Percy insists that Tyson is not the monster everyone assumes.

Percy tries to distract himself with camp activities. Silena Beauregard teaches Percy to ride a pegasus, but the pegasi are terrified of Tyson and their reaction makes Tyson cry. Beckendorf becomes Tyson's one clear friend and takes him to the forge, where Tyson proves gifted with metalwork. Percy also keeps unofficial border patrol shifts and watches dryads and satyrs struggle to hold back the poison in Thalia's tree, which deepens Percy's anger at Luke for causing the camp's crisis.

The night before the race, Percy and Tyson finish a Poseidon-themed chariot together. Tyson notices that Percy is upset, calls himself a monster, and tries to promise he will be a good one; Percy admits that he is overwhelmed by Tyson's sudden place in his life, by the camp's danger, and by his growing fear that Grover is in trouble. That night Grover appears in Percy's dream from a cave and explains that the Cyclops Polyphemus has trapped him on an island in the Sea of Monsters after satyrs were lured there by a powerful nature scent that seemed like Pan. Grover also reveals that their empathy link now ties Percy's fate to his own, making Grover's rescue urgent.

On the morning of the race, Percy tries to tell Annabeth about the dream, but Annabeth suspects Percy is using Grover to distract her until Percy suggests consulting the Oracle. The race begins with crashes and sabotage from the other teams, and Percy and Tyson hold their own despite attacks from Beckendorf's chariot. Then thousands of Stymphalian birds with bronze beaks descend on the spectators and racers. When Annabeth recognizes the monsters and realizes the race no longer matters, Percy turns his chariot around and Annabeth joins him so they can find a way to save the camp.

Percy and Annabeth race to the Big House, grab Chiron's boom box and music, and blast Dean Martin across the track. The horrible noise disorients the Stymphalian birds and drives them into the open sky, allowing Apollo's archers to shoot most of them down and end the attack. Although Percy and Annabeth help save the camp, the grounds and chariots are badly damaged, many campers are wounded, Clarisse is awarded the victory because she crossed the finish line first, and Tantalus immediately prepares to punish Percy for disrupting the race.