Following the events of Pulse Point, 12-year old Ama and the other Underlanders living in a world of darkness dig for brine, the City’s real energy source. Underland is a dangerous place: cave-ins, starvation and illness areconstant 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.

For Kaia, setting foot outside can be deadly. Climate change has made Earth too dangerous for people to live anywhere but inside the City. Run on the energy created by its Citizens and sustainable in every way, the City uses microchips, called pulse points, to control the energy its inhabitants use. When Kaia's pulse point malfunctions, she experiences freedom from the strict City rules for the first time in her life. With unlimited energy, she doesn't need to run for hours on a sensor mat, and she can share energy to hide her grandmother's failing health. But a shocking discovery makes her question what goes on inside the City walls and sets her on a journey to discover the world outside them.