Keith is a burned-out cop who knows every move before the killer makes it, except the killer he's chasing doesn't follow any rules that exist yet. Koku exists in a different story entirely, one where redemption and damnation get tangled so tight you can't separate them. The show splits itself in half between a procedural that plays fair and a supernatural descent that doesn't, daring you to figure out which half is lying.

Keith Flick, a seasoned but eccentric detective sidelined to the Archives Department after a personal loss, is called back by the Royal Investigation Service of Cremona, a technologically advanced island nation. A vigilante known as Killer B, armed with a blade and superhuman abilities, is dealing out his own justice, praised by some and hunted by others. As the crimes grow bolder, it becomes clear that more than one culprit is at work. With his impulsive partner Lily Hoshina, and unexpected help from Killer B himself, Keith uncovers secret organizations, domestic terrorism and human experiments. Meanwhile the paths of Koku, the protagonist, and a mysterious criminal group cross his own. This psychological, supernatural thriller soon raises doubts about government corruption and about which allies can be trusted.