In the modern metropolis of Shipden, all is not well. At the heart of the city lies an ancient forest known only as The Sunken Garden, and any who enter are never seen again.

Amelia is living alone in the mountains and doing fine...or so she says. When the cheerful flying courier Diana crash lands onto her doorstep with a rescue mission, the duo is pulled into the garden - the one place Amelia swore she'd never be stupid enough to go.