When Ember Jones and her little brother Theo find themselves trapped in a forgotten underground world on the brink of civil war, their troubles are only just beginning.

As a key figure in an ancient prophecy, Ember becomes a coveted prize, desired by both factions. When Theo is brutally torn from her side, Ember summons the courage to embark on a perilous mission, where the stakes are nothing short of life and death. No sacrifice is too great, as she confronts impossible odds and faces the malevolent forces that stand between her and her brother.

With the help of the mysterious Sunday, can she defy fate, save her brother from the clutches of the corrupt Three, and find a way to lead them both safely back home?