Asura Amashiro, a second-year student at Izukoshi City's Dokka Middle School, lives by his late mother's words—those who do bad will be punished by the heavens, while those who do good will be rewarded. With this in mind, Asura tries to do one good deed daily. Even when subjected to brutal bullying by his classmates, he does not waver from his conviction.

One day, Asura comes across an old man at a street stall. The old man offers him a key, claiming it holds the power to send people to hell. Asura rejects the offer and returns home, only to find his dog being mutilated by his bullies. Their attention soon shifts to Asura himself when suddenly the key appears on the ground.

With his suffering being ignored by all, Asura decides to become the enforcer of divine retribution and uses the key to condemn the leader of the bullies to hell. Following that incident, he continues to use the key to perform good deeds and punish those who harm innocent people, unworried about what violent fate awaits them.