As newly inducted members of Japan's Narcotic Control Department, Haru and Kai find themselves up against a slew of obstacles, ranging from drug lords and criminal factions to homicidal maniacs, shadowy syndicates, and unpredictable individuals. Despite his kind-hearted nature and sensitivity, Kai struggles with his own personal demons and undergoes a drastic transformation into a highly skilled assassin at the slightest provocation--as if a switch has been flipped. As they go about their daily duties, the secrets of Haru and Kai's pasts, as well as the reason behind Kai's enigmatic behavior, gradually come to light.