At the end of The Lost Hero, we have a good idea of where the missing Percy Jackson is now. The Son of Neptune rejoins Percy at Camp Jupiter, where he has made some new friends: Frank Zhang and Hazel Levesque. When the three embark on a dangerous journey to Alaska, they have a chance to regain their cohort's honor - or to die trying.
Hazel is a daughter of Pluto who's keeping some dark secrets to herself. Hazel's secrets will be revealed before the group confronts a powerful Giant. Frank is a nervous kid whose life is tied to a piece of firewood. He knows his family had a powerful inherited gift, but Frank doesn't even know what the family power is, much less how to use it.