Fifteen-year-old Barnabas Bopwright is proud to live in the greatest city on earth. But under the shiny exterior lurks the hidden cost of the city's skyscraping success.

On his way to school, Barnabas discovers a transit map with an unknown subway line. Soon he's on an impossible train, running from authorities who have guarded the city's dark mysteries for a century. The subway brings Barnabas to a hidden valley outside the city where he's drawn into a terrifying world of deception and revenge.

In the valley, Barnabas discovers new friendships, a death-defying circus, an evil emporium, and a surprising attraction for another boy. But when his investigations uncover a terror plot to destroy the city he loves, Barnabas realizes it's up to him to save his home and bring deadly secrets into the light before it's too late.