"A Letter to Momo" is a poignant and supernatural coming-of-age story centered on Momo Miyaura, an 11-year-old girl grappling with the sudden loss of her father. After his death, Momo and her mother, Ikuko, relocate from Tokyo to the remote island of Shio, her mother’s childhood home. The move is intended to provide a fresh start, but Momo struggles with unresolved grief and guilt, particularly over an unfinished letter she had begun to write to her father before his passing. The letter, which only contains the words "Dear Momo," becomes a symbol of her inability to express her feelings.

Upon arriving on the island, Momo discovers a strange and weathered book in the attic of their new home, which seems to have a connection to her father. Soon after, three mischievous and otherworldly yokai—Iwa, Kawa, and Mame—manifest in her life. These supernatural beings, invisible to others, are bound to the island and tasked with observing humans. Initially disruptive and chaotic, the yokai gradually form a bond with Momo, offering both comic relief and emotional support as she navigates her grief and adjusts to her new environment.

The narrative unfolds as Momo interacts with the island’s tight-knit community, including her grandmother and other locals, who provide a sense of belonging and stability. The yokai’s antics often lead to humorous and heartwarming moments, but their presence also serves as a catalyst for Momo’s emotional growth. Through her interactions with them, she begins to confront her feelings of loss and guilt, ultimately finding the courage to complete the letter to her father.

The story reaches its climax during a dramatic storm, where Momo and the yokai must work together to save the island from a supernatural threat. This event not only strengthens Momo’s bond with the yokai but also symbolizes her acceptance of her father’s death and her ability to move forward. By the end, Momo’s journey reflects themes of healing, resilience, and the importance of community, as she finds closure and a renewed sense of purpose. The yokai, having fulfilled their role, depart, leaving Momo with cherished memories and a deeper understanding of herself.