Percy Jackson arrives at Goode High School for freshman orientation, already worried that his usual demigod troubles will get him expelled before the school year begins. His mother hopes he can make a good impression because her boyfriend, Paul Blofis, teaches there and helped get Percy accepted. Percy also knows he is supposed to meet Annabeth afterward, though he insists it is not a date.

Before entering, Percy spots Rachel Elizabeth Dare, the mortal girl who once saw through supernatural disguises at Hoover Dam. Trying to avoid her, Percy uses a side entrance, where two cheerleaders, Tammi and Kelli, greet him with suspicious friendliness. Kelli’s leg makes a metallic sound when Percy bumps it, confirming that something about the girls is wrong.

Inside the gym, Rachel finds Percy and demands to know why he is there. When Tammi and Kelli begin leading the orientation, Rachel reacts with terror and urges Percy to run. Percy follows Rachel to the band room, where she explains that she can see the cheerleaders’ true forms. Percy tells Rachel about the Mist, Greek monsters, the gods, and finally admits that he is a half-blood.

Tammi and Kelli trap Percy and Rachel in the band room. Tammi reveals herself as an empousa, a servant of Hecate who feeds on young men, and attacks Rachel. Percy destroys Tammi with Riptide, but Kelli transforms too, proving faster and more dangerous. Kelli warns Percy that Camp Half-Blood will burn and his friends will become slaves to the Lord of Time.

When a tour group approaches, Kelli manipulates the scene so Percy appears to be attacking her. She bursts into flames in front of Paul Blofis and students, setting the school on fire and making Percy look responsible. Rachel urges Percy to flee and promises to invent a story, while Percy escapes through the broken window.

Outside, Percy runs into Annabeth, who is upset to see Rachel chasing after him and to learn Percy told a mortal about half-bloods. Rachel writes her phone number on Percy’s hand and demands answers later. With police sirens approaching and Kelli’s warning still troubling him, Percy and Annabeth decide they must go to Camp Half-Blood immediately.