The Lost Hero
Jason, Piper, and Leo battle Enceladus
Piper had been keeping a secret about Enceladus telling her what to do. When she finally tells what has been happening, they plan to defeat Enceladus. When they get there, they find Piper's dad trapped and plan to save him while attacking by Piper getting him and Leo and Jason battle. Of course it doesn't go as planned when Enceladus sees it coming and has them all beat. When all seems lost Jason charges him and with Zeus's help, he kills Enceladus. That is a major part of the book because it shows that even though Piper almost tricked Jason and Leo to their deaths to save her dad, they had true friendship and still helped her save her dad.
In The Lost Hero, Jason wakes up on a school bus with two of his "friends" yet he has no memory. After being attacked by storm spirits, Jason, along with Piper and Leo (his friends), are taken to a camp for kids like them, demigods. At the camp, they get an explanation and shortly after are called to go on a quest to save the queen of the gods, Hera. The three have many battles and struggles along the way and even battle a giant but eventually they find and free Hera. Over the long journey Jason starts slowly getting his memory back and the group finds out about a lot of things that have and will happen that they must stop together.