Ten years ago, a bizarre phenomenon appeared in Tokyo. Three days later, two types of humans with supernatural abilities appeared: emotionless dolls that only do what they're programmed to and wield the ability of surveillance, and powerful contractors, feared and hated by the humans that know of their existence. Rationally self-preserving and without the feeling of guilt, it's not long before contractors are used as ideal assassins and weapons of war.

Unable to analyze the phenomenon that created these beings safely, it's decided that a huge wall be built around it, called "Hell's Gate." PANDORA, an international research group, protects this wall and analyzes what little they can, while the government struggles to suppress dangerous contractors and the publics' knowledge of them.

In all the police force's efforts and doll-assisted surveillance of the city, one contractor continues to evade them. BK201, as the police force knows him, stands out as an exceptional killer, while those in the syndicate who know him as Hei see him as a powerful, yet strangely irrational contractor...

Summary written by Quaenorde.

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Sequel: Darker Than Black: Ryuusei no Gemini