Following the death of her father Kazuo, Momo Miyaura and her mother Ikuko travel from Tokyo to the Seto Inland Sea. Momo carries Kazuo's unfinished letter, which contains only the words "Dear Momo". At her mother's estate in Shio Island (汐島, Shiojima), they meet their relatives Sachio and Sae Sadahama, and Koichi, a postman and an old friend of Ikuko, who has always had a crush on her. Momo is devastated and misses Tokyo. In the attic, she opens a present containing a rare picture book about goblins and Yōkai, collected by Sachio's father. Three droplets from the sky enter Ikuko's estate and transform into yokai consisting of Kawa, Mame, and Iwa, the group's leader.