When fifteen-year-old Charles Wallace Murry shouts out an ancient rune meant to ward off the dark in desperation, a radiant creature appears. It is Gaudior, unicorn and time traveler. Charles Wallace and Gaudior must travel into the past on the winds of time to try to find a Might-Have-Been-a moment in the past when the entire course of events leading to the present can be changed, and the future of Earth-this small, swiftly tilting planet-saved.