39-year-old Shintaro Ojiro is what society labels a lower-class citizen: uneducated, underemployed, and entirely isolated. He has been stripped of everything and sees no path forward. In his despair, he decides that assassinating a politician is the only way to become a hero and change the country. He trains alone in his squalid apartment, crafting crude weapons and explosives, and sets a date to carry out his plan.

His focus is broken by a neighbor girl, Yotsuba, who is being neglected at home. Against every instinct of a man who wants to be left alone, Ojiro starts feeding her. When loan sharks connected to the Kurobukai yakuza syndicate kidnap Yotsuba as collateral, Ojiro abandons his political target and goes after her instead. Armed with his homemade gear, he storms the yakuza den and, to his own surprise, massacres the gangsters. In that first act of violence, he discovers a brutal talent for killing.

After saving Yotsuba, Ojiro leaks evidence from the dead men's phones onto social media, drawing the full attention of Kurobukai. It marks the beginning of an escalating war between one amateur vigilante and a vast criminal network. Word spreads through the underworld of a ghost called Ojiro, a terrifyingly effective killer who operates without rules.

The fight brings Ojiro into the orbit of Genpei, the estranged son of the Hiame-gumi family. Genpei despises the yakuza life and lives quietly in a high-rise with a stray cat colony and a runaway girl named Mitsuki. He surrenders to Ojiro and offers shelter. But Genpei's older brother Ryudai, the Hiroame-gumi underboss, is dangerously possessive and has Mitsuki kidnapped to lure out Genpei and the unknown attacker. Ryudai is a monstrous fighter, but Ojiro exploits his severe cat allergy to win the fight, though he spares his life.

Ojiro, Genpei, and Mitsuki go into hiding, but new trouble arises. Raimu, a child soldier trained in a youth facility operated by the Hiame-gumi, becomes obsessed with Ojiro's fighting style. When his superiors discover Raimu imitating the vigilante, he asks Genpei for help. Ojiro and Genpei go to rescue him from a human-trafficking hotel run by the Oshida-gumi, the Kurobukai subsidiary controlling the sprawling red-light district of Tenmotsuzaka.

The rescue turns into a prolonged war inside the hotel. Ojiro and Raimu manage to free the captives and escape, but Ojiro is betrayed and stabbed by a woman they were trying to protect. Wounded and captured, he is taken by the cool and calculating Oshida underboss Go Masato. Go uses the children as human shields to keep Ojiro pinned. Genpei and Raimu pursue in a car driven by Mitsuki, using homemade bombs to create chaos, and Ojiro breaks free. Go, comparing Ojiro to an unstoppable boar from his childhood, chooses to prioritize the children and lets Ojiro go. The battle ends with Tenmotsuzaka falling under Ryudai's control after Go's defeat.

Ojiro, Genpei, and the children they have saved settle into a quiet life in a new hideout. A mysterious woman arrives with an unknown agenda. Ojiro's focus has shifted from assassinating one politician to dismantling Kurobukai entirely. He continues to drift into ever deeper territory, using escalating violence to strike at the underworld, earning a reputation not as a hero, but as a monster.