Ikuko Miyaura is the mother of Momo Miyaura. After her husband Kazuo dies unexpectedly in a boating accident, she leaves Tokyo with Momo, relocating to her family's estate on Shio Island in the Seto Inland Sea. This move, seeking a fresh start, strains their relationship as Momo resents the upheaval from their life.

On the island, Ikuko reconnects with relatives Sachio and Sae Sadahama, and Koichi, a local postman and childhood friend who has long held feelings for her. To rebuild their lives, she takes nursing classes, often absent during the day, contributing to Momo's isolation.

Ikuko navigates her own grief and single parenthood. Conflict erupts when Momo accidentally breaks Ikuko's cherished mirror while trying to stop goblins from stealing produce. During the ensuing argument, Momo reveals her last, angry conversation with Kazuo before his death, forcing Ikuko to confront her daughter's unresolved pain and guilt.

Searching for Momo after their fight, Ikuko suffers a severe asthma attack. This life-threatening crisis requires Momo to seek help. Though the goblins decline direct aid, their indirect intervention enables access across the newly completed bridge, facilitating medical assistance. Ikuko recovers.

Reconciliation occurs during the tōrō nagashi festival, a lantern-floating ceremony honoring the deceased. Sharing this experience allows them to process their grief together. They realize Kazuo's unfinished letter, beginning "Dear Momo," contained his expression of pride in their daughter. This signifies mutual healing and acceptance, enabling them to move forward on the island.