Two young high school boys, Takeru and Ryo, are struggling through teenage angst when a mysterious light in the woods offers forth a young girl practically out of Takeru's dreams. They are promptly attacked by a strange alien-looking monster, which is defeated when Takeru mysteriously and spontaneously transforms into a powerful and strange looking beast himself. Not much is clear about the girl, who they name Hikari (Light), but she is more than a little bit strange. Without a memory or a past, the girl is integrated into the lives of the two boys and their families. She knows only her love for Takeru. Most of the group live in a type of group home, along with Jennifer Portman, a drunken American super-scientist who can't keep her hands off the boys and has her own theories about what Hikari is. The series follows them as they hang-out, go to school, and take trips, and all the while Hikari, and also later her "sister" Akari, learn about the human condition. They are also joined by a shape-shifting robot called Ioneos, apparently loyal to Hikari, and Kuon, a little floating alien. They also encounter more strange monsters on occasion. The series spirals towards a rather unexpected climax questioning the value of human life, relationships, love, and the spirit of survival shown by all life on Earth.