On his sixteenth birthday, orphan Yuuto Amakawa is approached by Himari, a sword-wielding cat spirit who tells him his family descends from the twelve Demon Slayer houses. Bound by an old oath, Himari moves in to guard Yuuto from the ayakashi now hunting him, a job his severe cat allergy makes no easier. As word of his bloodline spreads, other spirits and a slayer named Kuesu turn up in his life, drawn by the protective charm his grandmother left him.