The earth's core overheats. From Yellowstone to Sakurajima, thousands of cubic feet of magma spews into a summer sky. Ash plumes, miles high, incinerate most of the earth's inhabitants. In a bunker system deep in the Salmon River Mountains, a small band of survivors form the Undergrounders; among them, sixteen-year-old Derry Connelly, and a young Septite homesteader, Jakob Miller, who stirs up feelings inside her she didn't know were possible.When the Undergrounders dare to resurface, they encounter a bleak and dangerous new existence, terrorized by gangs of cutthroat Rogues, and the mysterious Sweepers who systematically target adolescents for extraction in ships that hover over the earth's magnetic fields.Derry's older brother, Owen, is one of the first to disappear. To save him from a fate more macabre than death itself, Derry must now become the leader she was destined to be and spawn a rebellion that strikes at the heart of the Sweepers' operation.