Kurose Jin was an ordinary high school student. Yet within him were memories of a past life, one where he was feared across another world as the "Sword Emperor".

One day, mysterious omens appeared worldwide. Hallucinations. Sudden illness. Cracks in the sky. As people scrambled, crying "Terrorism?!" or "Mass hysteria!", Jin alone knew the truth.

"--It's here. Dungeonification. X-Day."

In peace-addled Japan, he was the only one preparing for what was to come--training his body, stockpiling supplies, crafting a brutal weapon from sharpened rebar. Then, at the fated noon, a massive hole tears open in the school grounds. Goblins and Orcs pour forth.

Police pistols were deflected. A Self-Defense Force helicopter was shot down. The school descended into a screaming hell. As his classmates wailed in despair, Jin rose with a sadistic grin.

"Now then--time to earn. Bonus stage time."

This is the story of a man with supreme knowledge and cold calculation who hunts monsters, seizes magic stones, and claws his way to the top through overwhelming force in a crumbling modern Japan.