Rick Riordan's "The Lost Hero" is a fantasy novel centered around three children who are demigods and their quest to save the Greek goddess Hera. In her plot to overthrow the gods, Gaea has ordered her underling, Aeolus, to capture Hera and hold her in his castle. This book is the first in the "Heroes of Olympus" series, which is a spin-off of the "Percy Jackson and the Olympians" series.
Before Hera was captured, she switched the main character, Jason Grace, with Percy Jackson, and both boys have amnesia. Percy is at the camp of Jupiter, Jason's father, while Jason is on a field trip to the Grand Canyon. Jason and his friends, Piper and Leo, are attacked, and their guardian, a satyr named Gleeson, is kidnapped. The trio is rescued after the fight by Percy's girlfriend Annabelle, who tells them that they are demigods. She takes them to a school for children like them.
At the school, the three main characters are joined by Jason's newly-discovered sister, Thalia, and the group sets out to rescue Hera. Jason, Piper and Leo are captured by Aeolus, but they escape to San Francisco where they fight the Giant Enceladus and free Piper's father, Zeus. The group then returns and successfully rescues Hera. During the fight, however, the giant Porphyrion escapes into a hole in the Earth.