As a first-year student of the renowned Hyakkaou Private Academy, Mary Saotome knows her future is set for a one-way cruise to the top...That is, until an encounter with an old-classmate-turned-housepet throws the newbie princess into the world of gambling! Can a normal girl like Mary make friends and survive debtless through only wit and luck?

In her house, Mei Onigami discovers a strange round creature that only says "chimi." Her sisters, Mutsumi and Hazuki, agree to let the being stay with them until its owner comes to claim it. However, as soon as one of them utters the word "hell," more creatures show up until there are 12 in total. The sisters think nothing of this until the critters create a portal that summons an envoy from Hell. Jabberwock the Eighth, a horned and fanged man covered in fur, has come to unleash a living hell on earth under orders from his boss. However, when living in the human realm proves to be challenging, the Onigami sisters allow him to stay with them. They also take in his "Chimimoryo spirits"—the round demons that preceded his arrival. Called Jigoku-san by the Onigami sisters, this messenger from Hell has a lot of work cut out for him if he wants to take over the world. But before that, he will have to survive the everyday hells of human life.

Fuuta-kun has never owned a cat before, but his younger sister, Pii-chan, moves into his house with the round gray cat Kyuruga. Inexperienced with cats, Fuuta-kun has no idea what to expect, and each night after he returns home is marked by odd and endearing surprises from his new pet.