Rent, the protagonist, was a perpetually Bronze-ranked adventurer in the remote regions.

Unfortunately, he encountered a formidable monster deep within a labyrinth, was defeated, and when he came to, he had become a Skeleton.

Agonized by the thought that he couldn't even enter a town in this state, he remembered the concept of monster evolutionary advancement. Aiming to at least acquire a fleshy body, he continues to defeat monsters in the labyrinth.