Following the sudden death of her father, 11-year-old Momo Miyaura departs Tokyo with her mother to relocate to a remote, aging island in the Seto Inland Sea. The sole keepsake she possesses from her father is an incomplete letter containing only the words "Dear Momo"—a source of lingering unease in her heart. In this unfamiliar, small town, Momo hesitantly attempts to acclimate to the antiquated wooden structures, quiet agricultural fields, and enigmatic, secluded shrines. One day, while searching the attic of her new residence, she uncovers a tattered picture book about youkai. After this find, peculiar events start occurring throughout the town, and Momo encounters three troublesome youkai. Momo e no Tegami chronicles the journey of a young girl as she grapples with adapting to her strange new existence and eventually reconciles with her father's cryptic letter.