Yuuji Sakai, the usually vacant and somewhat weak protagonist, lives an ordinary life until the sudden appearance of a monster disrupts his ordinary life. Under the setting sun, Yuuji is suddenly attacked by an alien monster that transforms human "existence" into spiritual fire and then devours it.

At this time, a girl who called herself "Flame-haired and Burning-eyed Slayer" appeared and rescued Yuuji from the monster's mouth. Yuuji Sakai named her Shana based on the name of her large sword.

She told Yuji that his life was about to end - it turned out that outside this world, there was another world called the "Red World". In order to realize their ambitions, the people there took away the world's original power, the "power of existence", from humans one after another, and Yuji was one of the victims.

After the Crimson Apostle took away the "Power of Existence",...