On his sixteenth birthday, orphaned high schooler Yuto Amakawa learns from a sword-bearing cat ayakashi named Himari Noihara that he is the last heir of the Amakawa line of demon slayers and that the protective charm from his grandparents has just expired. Himari moves into the house Yuto shares with his childhood friend Rinko Kuzaki, sworn to guard him from the ayakashi now coming for his blood. Yuto's cat allergy makes the arrangement immediately complicated.