The book opens with Percy having a dream about his friend Grover. In the dream, Grover runs for his life down a deserted street in Florida. Percy identifies the thing chasing Grover as a monster because it smells “like a skunk that’s been living off Mexican food” (3). Grover takes cover behind a row of bridal gowns in one of the stores, and Percy wakes right after the monster breaks down the store’s front wall.

Percy lives in an apartment with his mom. It’s his last day of seventh grade, and tomorrow, he’s supposed to leave to spend the summer at Camp Half Blood, the summer camp for demigod children of the Greek gods. Over breakfast, his mom tells him she received a message from Chiron (activities director at camp), who said it may not be safe for Percy to return because the camp is having problems. She promises to give Percy the rest of the details after school. Outside, Percy sees “a shadow that belonged to no one” on the building across the street (7). It disappears but leaves Percy feeling uneasy.