Asura Amashiro is a middle school student who lives with his grandmother after his mother was murdered. He clings to his late mother's teaching that evildoers will always receive divine punishment and good deeds will be rewarded, striving to smile through the relentless bullying and violence he endures from his peers. Despite his innocent appearance, the trauma and his helplessness against cruelty have left him deeply broken inside.

One day, a mysterious old man gives Asura a key that is claimed to open the gates of hell. The man explains that by inserting the key into the head of someone he wishes to punish and turning it, that person will be sent to hell. Asura initially does not believe the story, but when a group of bullies kill his dog, the last gift from his mother, he reaches his breaking point. In a moment of desperation and rage, he uses the key. To his shock, it works. After witnessing the key's power, Asura decides to take divine judgment into his own hands, dispensing punishment to those who commit evil.

The setting is a contemporary Japanese city, revolving around Asura's school life and the urban environments where he encounters his targets. The narrative follows Asura as he uses the hell key to punish wrongdoers, from school bullies to more serious criminals. As he continues, he gains both allies and enemies. Other victims of abuse and trauma are introduced, including a female classmate named Otori Midori who lives with her mother and is bullied for her looks, and a transfer student named Tatsumiya Raika who bears emotional scars from his father. Both initially question Asura's sanity and methods but eventually grow to become his disciples, seeking to use the key's power to rebel against their own suffering.

As the number of disappearances rises, the police begin investigating the strange occurrences. Asura also discovers a significant cost to his actions: each time he uses the key, a permanent mark grows on his body, raising the question of how many more punishments he can deliver before the toll becomes too great. The story explores the moral dilemma of whether condemning people to brutal deaths that often outweigh their crimes is truly justice or merely delusion. The villains Asura targets are frequently portrayed as one-dimensionally evil, emphasizing cruelty over moral complexity.

Over time, the chaos escalates as demonic entities appear whenever the key is used, leading to violent and grotesque scenes where victims are dragged into hell. Asura's initially static but unpredictable character confronts the erosion of his hope in human goodness, with the boundary between justice and revenge dissolving rapidly. The plot deepens as the key's power spreads to other troubled individuals, each dealing with their own abuse and trauma, and the story questions whether a broken child wielding hellish power can ever truly be a force for good without losing himself entirely.