When her mentor goes inexplicably missing, gargoyle cadet Tiffany Noboru and her best friend Hedika Greene put their ghost hunting skills to the test in a harrowing journey across the underworld. To find him, they must brave what lies outside the Hollow Mountain and beyond the protection of their walls, but avoiding ghosts on the Endless See is only the beginning. Strange and dangerous creatures roam the blighted landscape between their home and their uncharted destination, the gargoyle sanctuary of Freefall, whose dying populace guards a terrible secret about her own world that some are willing to do anything to protect.