A legendary hitman known only by the codename Fable is ordered by his boss to take a year off from contract killing. Having raised the prodigy since childhood, the boss fears Fable has become too well-known and needs to disappear from the underworld to avoid attracting police attention. Fable is given a strict condition: he must live as an ordinary citizen and cannot kill anyone during this sabbatical, on pain of death for himself and his partner.

Fable moves to Osaka with his female assistant, who poses as his sister Yoko Sato. He adopts the civilian identity of Akira Sato, and the pair are given a house owned by the local Maguro Group yakuza clan. Their most difficult assignment is not assassination but surviving a normal life, a challenge for a man whose only skills involve violence and who struggles to understand basic social cues. While Fable can eliminate any target within six seconds and remain completely deadpan after a kill, he has no idea how to hold a conversation, find employment, or explain that his hobby is watching a low-budget local celebrity named Jackal. Yoko is his opposite in demeanor, a gleefully chaotic woman who spends her nights in bars toying with and humiliating unsuspecting men, though she is nearly as deadly as her brother when necessary.

The Maguro Group's second-in-command, Takeshi Ebihara, views the legendary assassin as a threat to the clan's stability and wants him gone. Constantly monitored by the yakuza and forced to navigate escalating provocations, Akira must outwit those who seek to expose or eliminate him while never resorting to his lethal training. He finds work as a bicycle courier after befriending his neighbor Misaki Shimizu, a hardworking woman holding down multiple jobs to pay off family debts. When a yakuza underling named Kojima attempts to blackmail Misaki into forced prostitution, Akira is forced to intervene while bound by his no-killing order, leading to a violent confrontation that nearly exposes his identity.

The second major arc introduces Rei Utsubo, a detective agency owner who appears respectable but hides a predatory nature. He has been the guardian of Hinako, a young woman in a wheelchair whose parents he murdered, and he repeatedly sexually assaults her while controlling her life. Utsubo becomes obsessed with identifying and eliminating Fable, putting both Akira and everyone connected to him at risk. Akira feels responsible for Hinako because his past assassination work may have caused the accident that injured her legs, and his attempts to protect her without breaking his vow of non-violence push him to the limit.

Throughout the year, Akira slowly learns what ordinary life means, producing childlike drawings that accidentally become profitable for his employer, surviving a mountain survival trip with a yakuza fanboy who worships him, and forming genuine connections with people who know nothing of his past. The tension between his instinct for ruthless efficiency and his commitment to non-violence defines the story, as Osaka's worst criminals keep forcing their way into his peaceful existence, each unaware that they are testing the patience of the world's most dangerous man.