Percy and Annabeth have just fallen into the deepest, evilest part of the underworld, Tartarus. The 6 other demigods on the Argo II must struggle on without them. Hazel, Jason, Leo, Piper, Nico and Frank must reach the mortal side of the Doors of Death to seal them to keep Gaea's armies at bay. The Roman demigod camp is almost ready to lay waste to the Greeks, and if they do, the war against Gaea will be a lost cause, and her armies will destroy the modern world as we know it. Leo and the others wonder if, when they seal the doors, Percy and Annabeth will be stuck in Tartarus forever. But the Roman army will be attacking in a month, and if they don't succeed, the gods will be destroyed right from their very roots.