While searching through their school's library, students Jin, Akira, and Rena discover an encyclopedia of monsters, originally owned by the Ahriman Occult. Rena explains the demons coexist with humans in a world called Valhalla, then takes notice of an incantation in the encyclopedia. As the three students recite the incantation, a girl named Ami summons them to a desolate Valhalla. Ami pleads for Jin and Akira, whom she claims are the Son of Light and Prince of Darkness, to save Valhalla with their latent abilities, as the world was torn by the desires of mankind and demons. Malicious Gargoyles suddenly appear and attempt to take special artifacts from Ami, until Jin and Akira fight back using gun-like Devilisers to summon the demons Rand and Devil. Their powers awakened, Jin, Akira, Rena, and Ami embark on a journey to save Valhalla from annihilation and find a way back home.
Jin, Akira, and Lena happen upon a book of demons in their school's library, which also tells of a world called Valhalla. Chanting a spell, the three are beckoned by Ami Kashihara to Valhalla, devoid of life, where Jin and Akira realize their true natures as the Son of Light and Prince of Dark. Joined by demons Rand and Gale, the group set off to find a way to restore Valhalla.