The novel opens as Percy Jackson rides with his mother and friends Annabeth and Thalia to a boarding school called “Westover Hall.” There, Mrs. Jackson drops the trio offer and they enter the school to look for Grover Underwood, a satyr who sent a distress call.

Once inside, they meet Vice Principal Dr. Thorn. Thalia uses the Mist, a magic force used to hide the existence of the Greek gods from mortals, to convince them that they’re students at Westover. They run into Grover at the school dance, who tells them that he’s found two demigods—students with one Greek god as a parent and one human parent—named Bianca and Nico di Angelo and that Dr. Thorn is a monster who is also hunting them. Grover explains that Dr. Thorn doesn’t seem to be sure that the di Angelos are demigods, and Percy realizes that the vice principal wasn’t fooled by Thalia’s trick. He is watching to see if the four of them will confirm that the di Angelos are children of a god.

Dr. Thorn stands near the di Angelos, guarding them, and Thalia suggests that they try to blend in. Percy and Annabeth start to dance together, and Annabeth tells Percy about how her father is moving to San Francisco.