In a battle against four overwhelmingly powerful races—angels, demons, undead, and humanoid beasts—humans managed to survive only through the heroics of Sid, who defeated the leaders of each race and locked their forces inside four pyramids. Centuries later, when Sid’s feats are mostly forgotten, Kai Sakura Vento serves as one of many guards watching over the pyramids, ever vigilant against the possibility of these ancient foes breaking free.
One peaceful day, while spending time with his friend Jeanne E. Anise, the world undergoes a sudden “overwrite,” propelling Kai into an alternate reality where humans are on the brink of extinction due to demon attacks and are forced into hiding. In this new world, Kai’s existence has been erased; none of his former friends, including Jeanne who now leads the local resistance, remember him.
Exploring the remnants of his old world, Kai meets Rinne, a mysterious young woman confined within one of the pyramids. Upon mysteriously acquiring Sid’s legendary sword, Codeholder, Kai frees Rinne. With a resolve to restore balance to this altered reality, Kai unites with Rinne and Jeanne to fight against the demonic oppression and save humanity. However, to return to his original world, Kai must first unravel the mystery behind the reality shift.