At the conclusion of The Mark of Athena, Annabeth and Percy tumble into a pit leading straight to the Underworld. The other five demigods have to put aside their grief and follow Percy's instructions to find the mortal side of the Doors of Death. If they can fight their way through the Gaea's forces, and Percy and Annabeth can survive the House of Hades, then the Seven will be able to seal the Doors both sides and prevent the giants from raising Gaea....

In The Son of Neptune, Percy, Hazel, and Frank met in Camp Jupiter, the Roman equivalent of Camp Halfblood, and traveled to the land beyond the gods to complete a dangerous quest. The third book in the Heroes of Olympus series will unite them with Jason, Piper, and Leo. But they number only six--who will complete the Prophecy of Seven? The Greek and Roman demigods will have to cooperate in order to defeat the giants released by the Earth Mother, Gaea....