In 2020, human warriors who possess supernatural abilities must battle deadly monsters to protect mankind from certain annihilation within a new dimension known as Mato. However, a boy named Shun Mizushino finds himself in a seemingly endless struggle for survival. One day, after narrowly surviving an overwhelmingly powerful double dungeon that nearly wipes out his entire party, a mysterious program called the System chooses him as its sole player and in turn, gives him the unique ability to level up in strength.