The Lost Hero is the first book in The Heroes of Olympus series by Rick Riordan. It introduces three new demigods: Jason, Piper, and Leo. Jason wakes up on a school bus with no memory of who he is, and he quickly learns that he's a demigod, along with Piper McLean and Leo Valdez. Together, they are brought to Camp Half-Blood, where they discover their divine parentage—Jason is a son of Zeus (Jupiter in Roman mythology), Piper is the daughter of Aphrodite, and Leo is the son of Hephaestus.

The trio is tasked with a dangerous quest to rescue Hera, the queen of the gods, who has been captured by a mysterious force. Their journey takes them across the United States as they face various mythological monsters and uncover clues about an ancient prophecy that ties together both Greek and Roman mythology. Along the way, Jason struggles with his lost memory, Piper grapples with her feelings of loyalty and betrayal, and Leo discovers his own hidden potential.

The book is filled with action, humor, and mythological references, introducing readers to a new set of heroes while expanding on the world from Riordan's Percy Jackson & the Olympians series.