At a young age, a part of Chiaki Mikado's soul was bitten away by a peculiar dog with three heads. Unable to feel happiness and joy as a result, he lives his days deprived of positive emotions, faking smiles when the situation calls for it.

Eight years later, his father sends him a box from Greece. Inside it is a girl with three forms and personalities: Kuro, Shirogane, and Roze. With the form of Kuro, she tells him that she will be Chiaki's guard dog. The girl reveals that she actually is the guardian of the Gates of Hades—the Cerberus—and is the same creature that tore his soul. But this time around, she will help Chiaki smile again.

However, this task is no small feat. Due to the missing piece of his soul, Chiaki is a magnet for fiends and wicked spirits. Meeting rivals, friends, and foes along the way, Chiaki perseveres to regain one of the most important things to being human—the capability to feel.