Seven years ago, Amakawa Yuuto's parents died leaving him living alone without a single relative. Ever since then, his childhood friend, Kuzaki Rinko, has woken him up, cooked for him and generally taken care of him. The only thing he has from any relative is a mysterious charm, a red ball, given to him by his grandparents as a good luck charm (an omamori).
From the day of his 16th birthday, his charm is said to not be able to protect him from evil spirits anymore, and Himari appears in Yuuto's life. Himari is a cat spirit-samurai girl and demon huntress who, aside from wanting to copulate with him madly, swears to protect Yuuto. There is only one small problem: Yuuto has a severe allergic reaction to cats.