In the mid-21st century, the discovery of ether perception gave humans access to supernatural abilities. Criminal organizations exploited this through illegal ether enhancement surgery, producing superpowered gang leaders who called themselves Demon Lords and seized control of Tokyo’s underworld. To counter them, the government legalized bounty hunting with a special drug called E3, which temporarily boosts a user’s ether powers; these sanctioned killers are officially known as Heroes, though in practice they are little better than the thugs they hunt.

Yashiro, a freelance Hero nicknamed the Reaper, lives a dissolute life of pizza, beer, and card games, preferring to work alone. One day he witnesses a schoolgirl fall from the sky and slam into the ground. The girl is Aki Jogamine, a trainee from the government-run Hero training academy. She had been pursuing a classmate kidnapped by the minions of a Demon Lord and was thrown off a building when overpowered. Reluctantly, Yashiro saves her, and she immediately begs him to become her teacher so she can rescue her friend. Against his better judgment, Yashiro agrees, and soon he is also approached by two other academy students: the socially inept Yukine Indou and the heiress Saera Kashiwagi Pendragon, who looks like a delinquent despite her refined background. All three insist on becoming his disciples, and Yashiro agrees to train them primarily for the money.

What follows is a gritty, street-level education in survival. Yashiro teaches the girls not heroic ideals but the cold, pragmatic techniques needed to stay alive in a world where morality is a luxury. Their first major mission takes them into the territory of the Demon Lord Amber Thorn Lord to rescue Saera, exposing the trainees to the full horror of real violence. Yashiro demonstrates why he is called the Reaper, using his ability to extend his subjective time—enabling hyper-speed perception and reflexes—combined with Western and Japanese swordsmanship. Aki, who can read minds, begins to grow as a combatant. The rescue succeeds, but it uncovers deep problems within Saera’s family and hints at corruption inside the academy.

As the story progresses, isolated incidents begin to connect into a larger pattern. The Demon Lords form an alliance and launch a coordinated assault on the academy itself, overwhelming inexperienced students. Yashiro disregards organizational orders and responds with ruthless efficiency, while his ally Ishinou, known as the Music Box, delivers brutal punishments that blur the line between Hero and villain. The young heroes hone their powers but pay heavy prices, and the conflict escalates into an outright war.

By the later volumes, Yashiro uncovers evidence that the very creation of the academy was a government experiment designed to serve the ether interests of powerful officials in league with the Demon Lords. This revelation shatters Aki’s belief in the system. Yashiro offers no comfort; he tells her that the difference between scum and real Heroes lies in what one chooses to do after learning the truth. The war culminates in a direct confrontation with one of Tokyo’s most formidable Demon Lords, the Storm Coffin Lord, whose spatial manipulation powers push Yashiro to his limits.

The series follows Yashiro and his three female students as they navigate a world where every institution is compromised and survival depends on knowing when to obey the rules and when to break them.