Sera has always felt as if she didn't quite belong among her people, the Cerulean, who live in the City Above the Sky. She is curious about everything, especially the planet her City is magically tethered to, and she can't stop questioning her three mothers, her best friend, Leela, and even the High Priestess. Sera has longed for the day when the tether will finally break and the Cerulean can move to a new planet. But when Sera is chosen as the sacrifice required to break the tether, she feels betrayed by everything she'd been taught to trust. In order to save her City, Sera must throw herself from its edge and end her own life. But something goes wrong. Sera survives the fall, landing on the planet below in a place called Kaolin. Sera has heard tales about humans, and she quickly learns the dangers her mothers warned her of were not just stories. Meanwhile, back in the City, all is not what it seems, and the life of every Cerulean may be threatened if Sera is not able to find a way home.