In a war against four overpowered races—angels, demons, undeads, and humanoid beasts—humanity could only prevail thanks to its champion Sid, who defeated the other races' champions and sealed away their armies in four pyramids. In an age where Sid's existence has become but a legend of the past, Kai Sakura Vento is one of the many conscripts tasked with monitoring the pyramids in fear that these old foes would escape.

On a peaceful day while spending time with his friend Jeanne E. Anise, the world is suddenly "overwritten," transporting Kai to an alternate reality where humanity has nearly been annihilated by the demons and forced into hiding. To make matters worse, his very existence has been erased from this world; all of his friends, including Jeanne, who leads the local resistance efforts in this world, have forgotten about him.

Trying to investigate the remains of his former world, Kai encounters Rinne, an enigmatic young woman trapped in a pyramid. Mysteriously inheriting Sid's legendary sword Codeholder, Kai manages to free Rinne from her imprisonment. Determined to fix this reality, Kai joins forces with Rinne and Jeanne to free humanity from demonic rule. But if the valiant soldier wants to have a chance at returning to his original world, he will first have to uncover the cause behind the mysterious alteration of reality.