Percy, the narrator and protagonist, hints at his involvement with an explosion at Goode High School during summer break. In the first week of June, 14-year-old Percy attends orientation at the new high school where his mom’s boyfriend, Paul Blofis, teaches. Percy uses a side entrance to avoid Rachel Elizabeth Dare, but two cheerleaders, Kelli and Tammi, stop him at the door. The cheerleaders have been waiting for Percy, which gives him an uneasy feeling.

In the gym, Rachel confronts Percy about their past encounter at the Hoover Dam, but yelps when she sees Kelli and Tammi. Rachel runs from the gym, urging Percy to follow, and the two meet up in an empty band room. Percy confirms Rachel’s suspicions that ancient Greek monsters and gods are real and reveals that he is a half-blood. Even though Rachel is a mortal, she can see monsters’ real forms through the Mist that hides them.

Kelli and Tammi find them and turn into their true form—empousai, vampire-like monsters with “weird, mismatched legs” (11). Using his sword Riptide, Percy slays Tammi but cannot defeat Kelli. She hints that the Titan army plans to invade Camp Half-Blood, and then pretends to be a victim, screaming for help when she hears a nearby tour group.