In Kaia's world, setting foot Outside can be deadly. The safety of the City has kept humans alive as the climate destroys the world; floods, famine, one disaster after another has kept people living in fear. But the City has found a way to survive sustainably: it is run by the energy generated by its Citizens. In the City, energy is the new currency and when someone can no longer produce enough, they are killed. Kaia is a Citizen, one of the few deemed genetically perfect enough to live under the protective dome of the City. The Prims, those who were not granted entry, fight for survival on the Mountain. But when Kaia discovers her mother is alive and living as a Prim, she escapes the safety of the City to find her and learns the truth about the Prims and the sinister world she left behind.