Annabeth and Percy are prisoners in the Underworld, on the Tartarus side, facing a thousand monsters. The five other demigods - Jason, Piper, Hazel, Frank, Leo - bury their disagreements and unite to find the entrance to the Gates, on the mortal side. Their mission: seal the Underworld and defeat the armies of Gaia. But victory will have a price... Percy and Annabeth would remain locked in the unwelcoming House of Hades.