Junpei Takeda, a meek high school student, regularly suffers at the hands of Kido Shouta and his gang of bullies. Luckily, his childhood friend Noriko Tatsumiya always comes to his rescue. One day, all of them are transported to a fantasy world by an aloof being who calls himself "God." Junpei quickly learns that he is just as weak in this new world as he was in his old one; yet, he now possesses a unique ability that allows him to learn a skill from any enemy he defeats.

After months of struggling to survive, Noriko and Kido trick Junpei into entering the "Sacrificial Labyrinth," a dungeon filled with powerful monsters that no one has ever escaped from. Driven by revenge, Junpei ventures deeper into the labyrinth in search of an exit.