Shirakiin Ririchiyo is a daughter of a prestigious family who has grown up isolated and subject to bullying from the servants and jealous schoolmates. She reflexively hides behind a pretentious façade and a sharp tongue that she regrets. To keep from hurting others and give herself time to change, she has decided to live alone in the Maison de Ayakashi residential tower where each resident is offered the protection of a private SS (Secret Service) bodyguard.

However, neither the residents nor the SS agents are what they seem. Each, including Ririchiyo, is an Atavist, a descendant of both human and Youkai (spiritually empowered beings native to Japan) who have inherited some of the Youkai's abilities and even memories. The residence acts as a secure haven for Atavists from pure-blood Youkai and other Atavist families.

When Ririchiyo moves in, despite declining an SS agent, Miketsukami Soushi introduces himself to her as her assigned agent and a descendant of a nine-tailed fox. Even though she has never before met him, he seems to know everything about her, claims that she once saved his life, and declares that he will be her faithful dog from this day forward.