The world is filled with cruel individuals, from petty criminals to sociopaths, who inflict suffering on innocent victims. When the anger of those victims becomes unbearable, they can call upon a mysterious woman named Houjou Shiori, the owner of the Uramiya, or the Revenge Shop. For a price, Shiori and her operatives will guarantee that clients get the revenge they deserve. The fifth part of the series, Evil Heart, introduces a formidable antagonist who challenges the Uramiya's mission. The story is set in contemporary Japan and follows a connected, episodic format where each client's revenge is typically resolved within a few chapters, while a larger narrative unfolds in the background.

The central conflict begins when a young man named Seya Kurasuke, acting on the advice of a mysterious figure named Evil Heart, posts a video online of a fatal prank. A woman dies after slipping on water-absorbing polymer pellets scattered on stairs during rain. The victim's father hires the Uramiya to get revenge, leading Shiori to execute a scheme that mirrors the original crime. However, the man calling himself Evil Heart is not just a random instigator. He begins to move behind the scenes, setting his own plans in motion.

The Uramiya team includes Shiori, a cold and calculating strategist who refers to herself as a necessary evil; her skilled hacker known only as the Informant; and her reliable enforcer, Takeomi Shiwasuda. A significant storyline involves the introduction of new characters. Detective Satoshi Kizuki becomes an ally to the Uramiya, but he is soon murdered after being attacked by two Chinese criminals, Kenpo Cho and Eiho Ryu. The detective's widow, Yoka Kizuki, seeks revenge from the Uramiya. In a parallel plot, the Okinawa branch manager of the Uramiya investigates a homestay fraud scheme. A young woman named Sakuragi Kizuki, the deceased detective's daughter, is a troubled delinquent who patrols the streets at night, violently punishing thugs. Shiori sees a potential operative in Sakuragi, but she is wary of the girl's love of violence, which mirrors the fate of a previous operative.

A major revelation redefines the conflict. The Informant discovers that the widow Yoka, manipulated by her lover, was the one who hired the Chinese criminals from the Earlier, the audience learns that a service called the Dark Hello Work Evil Heart facilitated the hit. When Yoka is later threatened by the same criminals, she hires the Uramiya for revenge, breaking the groups rules by revealing their existence. Shiori turns Yoka into a target, which creates a tense standoff that leads Sakuragi to learn the truth about her mothers dark secrets.

Throughout these arcs, the mysterious Evil Heart is systematically investigated. He is revealed to be a genius hacker and a man with a lizard tattoo on his neck. He does not commit crimes himself but provides advice, money, and connections to malicious people, encouraging them to commit evil acts for his own entertainment. In a recurring subplot, Sakuragi learns that her childhood friend, Sumiyo Saisoku, committed suicide after being stalked by the man with the lizard tattoo, confirming that Evil Heart has been connected to the Uramiya's world for years. The narrative builds toward a direct confrontation between Houjou Shiori and this shadowy figure who runs his own twisted version of a revenge agency. Cases in the series include punishing a drunk driver who let an innocent man take the fall, exposing a wife who physically abuses her husband, and dealing with a delusional woman who terrorizes a bookstore manager. Each case serves as a piece in the larger puzzle, illustrating the methods of both the Uramiya and the rising threat of Evil Heart.