Percy grapples with his heritage and his complex relationship with Tyson, learning to overcome prejudice.

Percy is joined by Annabeth Chase and his newly discovered half-brother, Tyson, who is a Cyclops.

The story begins with Percy's seventh-grade year ending in a "mythic disaster" after a monster attack at his school. Upon returning to Camp Half-Blood, he discovers that the magical borders protecting the camp have been poisoned. To save the only safe haven for demigods, Percy must venture into the treacherous Sea of Monsters—known to mortals as the —to retrieve the mythical Golden Fleece .

The importance of relying on friends to overcome impossible odds.