On Arborium, the last forested island in the future, fourteen-year-old Ark, who lives in a mile-high city set in the tree tops, overhears a plot to rob Arborium of its wood, a natural resource more precious than gold.
Sabrina, contacted by a desperate ghost, enlists her friends Megan, Connie, and Daisy to help her mount an ambitious fundraiser in an attempt to keep the dying Balm of Gilead tree in which the ghost girl lives from being cut down.