"The Mark of Athena" is the third book in the Heroes of Olympus series by Rick Riordan. It follows the adventures of demigods Percy Jackson, Annabeth Chase, and their friends as they embark on a quest to unite the Greek and Roman demigods. The story revolves around the search for the Mark of Athena, a powerful symbol that leads them to the lost Athena Parthenos statue, which is crucial for preventing a war between the two camps. The narrative explores themes of friendship, loyalty, and bravery as the characters face various challenges, including mythical creatures and ancient prophecies. The book combines elements of Greek and Roman mythology, showcasing the characters' growth and their struggle to overcome personal fears while working together to save their world.