Following the death of his parents, Yuuto Amakawa leads a fairly ordinary life in the city. The only issue he needs to worry about while attending school with Rinko, his next-door neighbor, is his cat allergies. Everything changes on his sixteenth birthday, when an Ayakashi—a supernatural creature—attacks him for the sins of his ancestors. Fortunately, he is rescued by Himari, a mysterious cat-woman wielding a sword, who explains that Yuuto is the heir to a family of demon-slayers. She is there to protect him now that the charm keeping him hidden from the world's supernatural forces has lost its power. Omamori Himari chronicles Yuuto's interactions with various forces of the supernatural world, as well as the growing number of women who appear at his doorstep, each harboring their own dark desires. Will Yuuto be able to adjust to his new "exciting" environment? Or will the ghost of his (ancestor's) past catch up with him?