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 from both sides and prevent the giants from raising

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"Matt Thorsen is a direct descendent of the order-keeping god Thor, and his classmates Fen and Laurie Brekke are descendents of the trickster god Loki. When Ragnarok--the apocalypse--threatens, the human descendents of the gods must fight monsters to stop the end of the world."--Provided by publisher

Demigod Percy Jackson, still with no memory, and his new friends from Camp Jupiter, Hazel and Frank, go on a quest to free Death. But their bigger task is to unite the Greek and Roman camps so that the Prophecy of Seven can be fulfilled.