After her father's unexpected death, 11-year-old sixth-grade elementary school girl Momoko Miyaura (voiced by Karen Miyama) moved from Tokyo to Shio Island in the Seto Inland Sea with her mother Ikuko (voiced by Yuka). Her busy mother had no time to take care of Momoko, which often made her feel lonely and helpless in the new environment. Momoko has always kept a letter from her father that only began with "To Momoko", but she could never understand what her father wanted to convey to her through the letter. When she first arrived at Shio Island, Momoko opened an old box in the attic containing a picture book depicting monsters, and began to notice a series of incredible things around her. "There must be something on Shio Island", Momoko, who holds this idea, will encounter in the strange and lively summer of Shio Island...? This film is produced by the well-known Japanese animation company ProductionI.G. It is a warm and healing work by the director of "Werewolf" Keiyuki Okiura after many years.