After the death of her father, along with her mother, the young teenage girl Momo is forced to leave her native Tokyo and move to live in a small town on the island of Shio, where her grandmother and grandfather live. Momo was never very active and sociable, and the tragedy made her even more withdrawn. Before death, her father was going to write a letter for Momo, but didn't have time. On a white sheet of paper there was only an inscription: 'Dear Momo'.
Frustrated by life in a new place where there is absolutely nothing interesting, Momo feels even more lonely. But over time, the girl begins to study the house, where she finds interesting and extremely rare books about the demons that her father once collected. In the absence of her mother, Momo begins to read these amazing books. Soon, the girl notices extremely strange and inexplicable things that frighten her, but this becomes the beginning of an amazing friendship with unusual friends.