There are six rules that were established so that humans and youkai can co-exist:
You must never harm a human.
You must never show your true form to a human.
You must obey the laws that humans have made.
You must never enchant a human.
You must never have children with a human.
You must never bring a human into youkai territory.
The Minatsuki family has existed peacefully in the human world and has prospered by obeying these rules. This is a story about Airu, the only daughter of the head of the Minatsuki family, the Minatsuki family butler, Raito, and his co-worker maid, Fuuka.
Contrary Airu can't honestly express her feelings and bosses people around. Fuuka has a stiff exterior but can't help teasing whenever she sees an opportunity. This is how they spent their peaceful, everyday life.
One day an incident occurs in the town where they live. Minatsuki Onizou, the head of the household, asks Raito and Fuuka to protect Airu. The two of them must now stay by her side day and night. They accompany her, not just inside of the mansion, but at school as well.