Despite his kind and selfless personality, high school student Hiiragi Yatsuse struggles to make friends with his peers. One evening on his way home, he stumbles across Tsumugi, an eccentric girl searching for her mother. Without much explanation, she quickly invites herself over to his house.

With Tsumugi's help, Hiiragi escapes an encounter with a supernatural creature that has appeared in his room. Tsumugi then reveals her true identity—she is actually a demon-like being called an oni. When Hiiragi recognizes Tsumugi's cherished amulet, he agrees to take her to the shrine where it originated from. Embarking on a journey to find Tsumugi's mother, the two meet new faces and overcome challenges along the way.