Heather is sleeping peacefully after the birth of her twin daughters when the sound of the world ending jolts her awake. Stumbling outside with her babies and her new husband, Brendan, she finds that their city has been destroyed by falling meteors and that her little family are among only a few who survived. But the mountain that looms over the city is still green--somehow it has been spared the destruction that has brought humanity to the brink of extinction. Heather is one of the few who know the mountain, a place city-dwellers have always been forbidden to go. Her dad took her up the mountain when she was a child on a misguided quest to heal her legs, damaged at birth. The tragedy that resulted has shaped her life, bringing her both great sorrow and an undying connection to the deep magic of the mountain, made real by the beings she and her dad encountered that day: Estajfan, a centaur born of sorrow and of an ancient, impossible love, and his two siblings, marooned between the magical and the human world. Even as those in the city around her--led by Tasha, a charismatic doctor who fled to the city from the coast with her wife and other refugees--struggle to keep everyone alive, Heather constantly looks to the mountain, drawn by love, by fear, by the desire for rescue. She is torn in two by her awareness of what unleashed the meteor shower and what is coming for the few survivors, once the green and living earth makes a final reckoning of the usefulness of human life and finds it wanting.