When Chiaki Mikado was young, a strange three-headed dog bit a part of his soul away, leaving him unable to experience happiness and joy. He goes about his days without positive emotions, feigning smiles when necessary to get by.
After eight years, Chiaki's father sends him a box from Greece containing a girl with three forms and personalities: Kuro, Shirogane, and Roze. As Kuro, the girl declares herself as Chiaki's guardian dog and reveals that she is the Cerberus, the guardian of the Gates of Hades—the very creature that took a piece of his soul. But this time, her mission is to help Chiaki regain his ability to smile.
Unfortunately, this is not an easy task. Because of the missing part of his soul, Chiaki attracts fiends and wicked spirits. As he meets friends, rivals, and foes on his journey, Chiaki must overcome these obstacles to reclaim the most important aspect of humanity—emotional capacity.