Yuuki Wakasa may be short-tempered when it comes to other people, especially her troublemaking younger brothers, but she is otherwise a normal fifth-grade student at Kamishiro Elementary School. One day, a small, red bean known as a magatama falls from the dark sky and into her hands. Though tempted to throw it away out of fear, she decides to keep the stone after recalling that it is a bad omen to do such an act, and so she confronts a vision of a strange woman in a magical mirror known as the Shinjyukyou later that night.

The woman warns Yuuki that her world is in danger and explains that recent occurrences of strange, dark weather came forth due to the disappearance of Amaterasu, the sun goddess. Chosen as the "Horizon Girl," Yuuki must now travel to the dream world with her new magatama and find the four other magatama bearers—the Amatsukami. Will Yuuki and the Amatsukami be able to save Amaterasu before the sun in both the spiritual world and their own disappears forever?