A near future, where humans have been defeated by the viral parasites named Gastrea. They have been exiled into a small territory and live in despair, side by side with terror. In this world trapped in darkness— Rentarou, a boy living near Tokyo and member of the "Civil Security"—an organization specializing in fighting against the Gastrea—is used to accomplishing dangerous tasks. His partner is Enju, a precocious young girl. They fight thanks to their peculiar powers until one day, when they receive a special assignment from the government. This top secret mission is to prevent the destruction of Tokyo... Set in a near future, this thrilling heroic-action story... starts now!