Manga adaptation of the classic video game of the same name.
The three worlds in this universe are Genjitsukai, the Human World, Mugenkai, the Dream World ruled by Valia, and Ankokukai, the Dark World ruled by Rogles. Rogles and his 5 dark lords are trying to take over the other two worlds, and have stolen a source of "light" power from Valia. Valia has responded by summoning a "Valis Warrior", who uses the power of the light within her, from the human world.
This Valis warrior is Yuko, who was an ordinary high school girl until she was pulled into Mugenkai. It's up to her to find a way to defeat the evil Rogles and retrieve the stolen power sources. But she has more than Rogles and his underlings to deal with, because Rogles has acquired a warrior of his own. This "Vecanta Warrior", who wields power by the darkness within her, is Reiko, Yuko's friend from the human world. But Reiko is not acting like herself, and seems to view Yuko as only an enemy. Will Yuko be able to both stop Rogles and find a way to return Reiko to the way she was before? Or will she have to seriously fight her friend?