A vigilante is loose on the Cremona islands. The infamous "Killer B," equipped with a sharp blade and superhuman ability, is hailed by some and persecuted by others. When the Royal Investigation Service (RIS) is unable to catch this outlaw, they enlist the help of Keith Flick, a seasoned but quirky detective who was sent to the Archives Department suffering a personal loss. As the crimes in Cremona get more complex, from covert deaths of wrongdoers to sophisticated assassinations of major people, it becomes evident that more than one individual is to blame. Keith begins to unravel conspiracies involving hidden organizations, domestic terrorism, and unexpected support from the elusive Killer B himself, with the help of his impetuous sidekick Lily Hoshina and unexpected help from the elusive Killer B himself.