After a magical circle suddenly appears at school, Hajime Nagumo and his classmates find themselves transported inside a church where they are welcomed as honored guests. Thrust into the RPG-like world of Tortus, each student shows great promise in both strength and never-before-seen classes—everyone besides Hajime. Given the class "Synergist," which boasts borderline average stats on top of being weak in combat, Hajime is soon considered to be useless by the rest of his class.

Working towards strengthening themselves, the students descend into a dungeon, but a mishap causes the entire class to be plunged into danger. Cornered by a large demon, they are trapped on a bridge looming over an abyss, the only way out being in front of them. In an effort to save his class, Hajime declares himself as bait, hoping his sacrifice will grant a chance of escape for the others. Though he intends to escape as well, a stray fireball flies in his direction, sending him flying off the bridge and down into the depths of the dungeon. Miraculously surviving, Hajime now begins his quest of survival in an attempt to reach the surface.