After the unexpected death of her father, 11-year-old Momo Miyaura leaves Tokyo with her mother and moves to an old remote island in Seto Inland Sea. The only memento she has from her father is an unfinished letter with only two words inside: "Dear Momo"—along with her heart's unrest from it. In the new and unfamiliar small town, Momo reluctantly tries to adjust to the outmoded wooden buildings, silent crop fields, and mysterious isolated shrines. One day, while exploring the attic of her new home, she finds a worn out picture book about youkai. Following this discovery, strange things begin to happen around town, and Momo is greeted by the arrival of three troublesome youkai. Momo e no Tegami tells the story of a young girl as she struggles to adapt to her bizarre new life and ultimately come to terms with her father's mysterious letter.

When Kasumi Toyama was out camping as a child, she heard the rhythm of the "Star Beat" while admiring the starry night sky. This sparkling, heart-pounding sound left a lasting impression on her, and since then, she has constantly been seeking to rediscover that exciting thrill. Now, as a newly enrolled student of Hanasakigawa Girls' High School, Kasumi is more determined than ever to find that long-lost sensation again. She attempts a myriad of school club activities, and even considers taking up a part-time job, but none of these feel suitable for her. That is, until she chances upon a star-shaped guitar in the storage room of an old pawn shop. It leads her to a live house, where she is captivated by watching a live performance for the first time. Spurred from the adrenaline, she instantly decides to form a band without even really knowing why. However, such a venture requires her to confront various hurdles with new friends she makes along the way, all in her pursuit to encounter that twinkling, heart-throbbing ardor once more.