Following the passing of her father Kazuo, Momo Miyaura and her mother Ikuko embark on a journey from bustling Tokyo to the serene Seto Inland Sea. Momo, burdened with an unfinished letter from her father that only bears the words "Dear Momo," struggles to adjust to their new surroundings at Ikuko's estate on Shio Island (汐島, Shiojima). There, they are welcomed by relatives Sachio and Sae Sadahama, along with Koichi, a postman who has long harbored feelings for Ikuko. Despite the warm welcome, Momo feels lost and nostalgic for her life in Tokyo.
As Momo explores the attic, she discovers a unique gift—a picture book filled with tales about goblins and Yōkai, a collection passed down from Sachio's father. Soon after, three droplets fall from the sky and morph into playful yokai: Kawa, Mame, and their leader Iwa. Their arrival coincides with changes in Momo's home life, as Ikuko enrolls in nursing classes, leaving Momo alone to confront the odd happenings around the estate.
One eerie day, Momo is startled by the mischievous yokai, who chase her out of the house. In her flight, she encounters a boy named Yota. While Ikuko and Yota dismiss the peculiar sounds as harmless, Momo becomes increasingly aware of the supernatural presence in her surroundings. The following day, Momo befriends Yota and his sister Umi. However, when a thunderstorm approaches, she seeks shelter, only to encounter Iwa, Mame, and Kawa again, who confess to stealing fruit from the island.
Momo's fear takes a turn when she learns that these yokai were once gods punished for breaking divine rules. Determined to protect her home, Momo tries to stop their vegetable heists. However, a misunderstanding leads to Kawa unintentionally shattering Ikuko's mirror, igniting a conflict between mother and daughter. In the midst of the chaos, a heart-wrenching memory resurfaces regarding her last argument with Kazuo before his untimely demise on the water.
When Ikuko's condition worsens due to a severe asthma attack, Momo realizes the gravity of her situation. Desperate for help, she implores the yokai to assist her in finding a doctor. Though they refuse, Momo takes it upon herself to search for a healer across the island. In her moment of need, Koichi and Yota pursue Momo, and she reveals the hurtful fight she had with her father before his accident, prompting Koichi to join her quest.
Eventually, the yokai discover that their escape from punishment lies in aiding Momo and Koichi, allowing them to cross the freshly completed bridge to find medical help. After a frantic search, Momo finally writes a heartfelt letter to Kazuo, expressing her gratitude as Ikuko begins to heal. With the mission finished, Iwa, Mame, and Kawa transform back into droplets and ascend to the sky.
That evening, during the traditional tōrō nagashi ceremony, Momo and Ikuko come together, reconciling their differences. They embrace the realization of Kazuo's pride in Momo. Now ready to embrace a new chapter, Momo swims joyfully with Yota and their friends beneath the bridge, stepping forward into the sunshine of a fresh start.
