Naze Boku no Sekai wo Daremo Oboeteinai no ka?

In an era where humanity triumphed over the five rival races with the help of the hero Sid, a boy named Kai suddenly witnesses the world being "overwritten." In this altered reality, humans have lost the war due to the absence of Sid, leaving dragons and demons to dominate the land. Kai, now a forgotten existence among humans, encounters a mysterious girl named Rinne, which sparks a resolve within him to change this rewritten fate.

Determined to restore humanity's victory, Kai inherits Sid's sword and martial arts. With Rinne by his side, he sets out to challenge the powerful enemy races that now reign supreme. In a world without a hero, Kai must rise to the occasion and fight for humanity's future.