When Silenus, Primas of the Satyr Clan, sends an urgent message to King Priam, it is the beginning of another adventure for Percy Fitz, Princess Portia, and Puma. It seems that the Helena Dome, another Netherworld kingdom, is on the verge of collapse. The only hope of saving the Satyrs and humans that live there is some maps made by a cave explorer named Carson Provost, and Percy's knowledge of the Outworld. Equipped with a portable Earth Sublimator ( a device capable of boring through a dome wall) the three companions set out on another journey. It's a journey fraught not only with danger from the natural world, but danger from their evil enemies, Manu the Manticora, and Glaucus the Satyr.