“Monster ‘Human’ killed. Congratulations, you have level up to 1!” Erika, a second-year high school student, heard a mysterious voice immediately after killing a classmate who had been abusive towards her with a knife. Feeling something strange, she leaves the place to find the school filled with corpses and monsters, turning into an inescapable dungeon. Having gained the fighting power through “leveling up,” people turn to Erika for help, and everyone begins to rely on her. On the other hand, in this place where “people die as a matter of course,” killing someone in revenge is not a serious crime. It was the biggest “chance” in her life, a life in which she had always been oppressed by others. At this school, which has become hell for the people and paradise for Erika, she sometimes saves people as a hero, and sometimes gets covered in blood as a revenger, fighting to escape from the school that has become a fantasy world.