In the end of the last book, The Mark of Athena, Percy Jackson and Annabeth Chase falls into the deep pit of Tartarus right after they got hold of the Athena Parthenon. So the 7 demigods of The Prophecy of Seven divides but meets back eventually. With the Athena Parthenon, they could either make a choice of saving the camps or to continue their quest and go to the House of Hades where they might be able to close the Door of Death, a door that connects to the deepest part of the Underworld. In Tartarus, two new big characters appear, a Titan and a giant with the names of Bob and Damasen. They both are friendly and helps Percy and Annabeth in surviving. I really liked the part about Bob because of his behaviour due to his lost memory. Until now, I think this book was the best book in the series of The Heroes of Olympus.