As the unicorns of Luster prepare for an invasion led by their ancient enemy, Beloved, a troubling new clue to the dangers they face is discovered in an ancient scroll. As a result, the queen must send her granddaughter - the human girl, Cara Dianna Hunter - to the Valley of the Centaurs, in quest of an ancient story that may hold the key to survival for all of Luster. At the same time, Cara's father, Ian Hunter, is searching for a way to enter the Rainbow Prison, in order to free Cara's mother from the strange spell that holds her. As father and daughter travel their separate paths, each will discover unexpected friendships, face heart-stopping danger, and uncover puzzling new mysteries. But it is Cara who will face the hardest choice of all - a choice that may determine the fate of Luster, and the unicorns she has come to love.