Percy and Annabeth race on Guido to intercept Rachel Dare's out-of-control helicopter, whose pilot has fallen asleep over Manhattan. Annabeth leaps onto the aircraft, Rachel pulls her inside, and Percy helps the injured Guido survive a dangerous crash landing. Annabeth, relying on scattered aviation knowledge, lands the helicopter safely, but tension flares when Rachel admits she came because she knew Percy was in trouble.

Rachel tells Percy that she has been drawing Greek symbols and receiving strange visions. Her message from the beach says that Perseus is not the hero, leaving Percy shaken and uncertain about the Great Prophecy. Chiron takes Rachel aside to discuss her visions, while Percy worries that her gift may be dangerous, remembering what happened to Luke's mother.

After finally sleeping, Percy dreams first of Nico arguing with Hades. Nico urges Hades to stop clinging to old grudges and help Olympus, but Hades refuses because of Zeus's role in Maria di Angelo's death and his long resentment of the Olympians. The dream shifts to Kronos at the United Nations, where Kronos orders Ethan Nakamura to unleash a Lydian drakon immediately, hoping to crush Olympus before Typhon arrives.

Percy wakes to the drakon's roar and learns from Rachel that the monster is fated to be killed by a child of Ares. Percy realizes Kronos chose the drakon because his spy knows the Ares cabin is absent. With the defenders exhausted, undersupplied, and nearly overrun, Percy, Annabeth, and Mrs. O'Leary attack the immense serpent while the enemy army pushes toward the Empire State Building.

The Ares cabin arrives in chariots under someone wearing Clarisse's armor, rallying the defenders and attacking the drakon. The disguised leader is poisoned while charging the monster, and the real Clarisse arrives with Chris Rodriguez, revealing that another camper had impersonated her to bring the cabin into battle. Enraged, Clarisse takes the fallen spear, blinds the drakon with an electric strike, and destroys it.

When Annabeth removes the poisoned warrior's helmet, Percy sees that the disguised leader is Silena Beauregard. Rachel's warning about a trick ending in death becomes clear: Silena's deception saved the battle by bringing the Ares cabin, but Silena has been mortally wounded.