In a world where everyone receives a skill upon reaching level 50, Shun was given the "Level Down" skill. The more he fights, the more his level decreases, a worthless skill in the eyes of everyone. Having trained tirelessly up until that point, he finds himself getting weaker by the second. Eventually, he is almost killed by a low-level monster, but when he defeated it with all his strength, he finds his level has dropped into the negatives. Shun, confused, obtained the power of "the other side of the world." In this world, levels itself are a curse. Shun, freed from this curse, gains access to abilities that defy this world's rules!

Humanity was attacked by monstrous beings known as Carriers, emerging from a tunnel that suddenly connected to another world. As if that weren’t enough, an unknown virus began infecting people, leading to widespread death. Despite being a Purifier tasked with hunting down Carriers, Kaito Saeki, a frail young man, fights on the battlefield, risking his life to earn money for his equally sickly younger sister’s medical treatment. However, his days are filled with both battle and bullying from his own comrades. One day, an unidentified new-type Carrier appears, leaving Kaito in a desperate, inescapable situation…! At the brink of death, refusing to give up on life, he suddenly encounters a mysterious girl who effortlessly wipes out the Carriers before his eyes. With a fearless smile, she makes him an offer. "Become my host."