When he was young, a peculiar three-headed dog bit away a portion of Chiaki Mikado's soul. As a result, he is unable to feel happiness or joy, living his days devoid of positive emotions and faking smiles when the situation demands it. Eight years later, his father sends him a box from Greece. Inside is a girl with three forms and personalities: Kuro, Shirogane, and Roze. In her Kuro form, she tells him that she will serve as Chiaki's guard dog. The girl reveals that she is actually the guardian of the Gates of Hades—the Cerberus—and is the same creature that tore his soul. However, this time, she will help Chiaki smile again. Yet, this task is no small feat. Because of the missing piece of his soul, Chiaki acts as a magnet for fiends and wicked spirits. Along the way, he meets rivals, friends, and foes, persevering to regain one of the most important aspects of being human—the capability to feel.