Momo Miyaura, a shy and withdrawn 11-year-old girl, moves to a remote island called Shio with her mother Ikuko after the sudden death of her father. Struggling with grief, Momo clings to an unfinished letter from her father containing just two words: "Dear Momo." As they settle into their new life with Ikuko focusing on social integration, Momo remains troubled by her father's ambiguous last message and finds it difficult to open up to the friendly islanders.

While exploring the attic of their new home, Momo discovers a dusty old book, inadvertently summoning three mischievous yokai (spirits) named Kawa, Mame, and Iwa. Initially frightened and annoyed by these spirits, Momo soon learns that they have a mission linked to her father's mysterious letter. Though they cause chaos and misunderstandings, the yokai gradually help Momo confront her feelings of loss and come to terms with her father's incomplete farewell. Through an adventurous journey filled with supernatural elements and heartfelt moments, Momo begins to heal, building new friendships and strengthening her bond with her mother.