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.” The two arrive at her mother’s estate in Shio Island (汐島 Shiojima?) and meet their relatives Sachio and Sae Sadahama as well as Koichi, a postman and Ikuko’s friend. 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.
When Ikuko begins to take nursing classes, Momo reads the book and begins to hear some strange sounds from the house. She is chased out of the estate by the yokai, only to encounter a young boy named Yota, who is Koichi’s son. Oblivious to the house’s strange noises, Ikuko and Yota assume that it is safe. The next morning, she meets Yota and his sister Umi. The three meet up with his friends and swim under the bridge, but Momo decides not to and runs to a shelter during a thunderstorm. Iwa, Mame and Kawa soon reveal themselves, having stolen some fruit from around the island. Frightened, Momo runs back to Ikuko’s estate and discovers that Sachio’s orchard was ransacked. Sachio then tells Momo that the yokai were originally gods, but they were transformed as a punishment for breaking the divine laws.
Momo attempts to prevent the yokai from stealing the local vegetables, only for Kawa to break Ikuko’s mirror. Momo’s failure causes an argument with Ikuko and she leaves. Shortly after, Ikuko suffers a near-fatal asthma attack while looking for Momo. Upon witnessing Ikuko being close to death, Momo realizes her mistake and asks the yokai to help look for a doctor on the other side of the island. However, the yokai decline and Momo leaves the house. Koichi and Yota pursue Momo, but she reveals her previous argument with Kazuo before his death and asks Koichi to help find the doctor. Meanwhile, the yokai realize that they are able to escape punishment by allowing Momo and Koichi to cross over the broken bridge and find the doctor on the other side of the road.
The next morning, Momo writes a letter to her father thanking him as Ikuko recovers. Having completed their mission to protect Momo, Iwa, Mame and Kawa transform back into the droplets and return to the sky. That night, Momo and Ikuko reconcile during the tōrō nagashi and the two eventually realize that Kazuo wrote that he was proud of her. Later, she begins her new life with Yota and the other children by swimming under the bridge.