Following the events of Pulse Point, 12-year-old Ama and the other Underlanders living in a world of darkness dig for brine, the Citys real energy source. Underland is a dangerous place: cave-ins, starvation and illness are constant threats. When their leader deserts them, Ama goes looking for him and stumbles on two Citizens. Neither knew the other existed. 18-year-old Sari lives in the City, blissfully unaware of what goes on beneath her feet. The disappearance of her best friend, Kaia, has left her feeling isolated. A chance encounter with her sister leads her to join a group called the Resistance who are on a secret mission to reveal the secrets the Council keeps hidden. When Kaia mounts an attack on the City, Sari and Ama are thrown together in a battle that will decide their futures and the fate of the City.