Following the passing of his parents, Shinpei Ajiro opted to live with the Kofune family and their two daughters, Mio and Ushio. He relocated from his island home to pursue further studies in Tokyo only to return when Ushio drowned while trying to save a little girl, in a tragic incident. While paying his last respects at Ushio's funeral, Shinpei's closest friend disclosed the presence of bruises on Ushio's neck, implying foul play, and provoking suspicion regarding the cause of her death. Shinpei initiates an inquiry into recent events and notes that peculiar happenings persist on the island. People are disappearing at an alarming rate, and Mio recollects a folktale she heard during her childhood about "Shadows," presumed to be supernatural beings. As per the legend, encountering one's Shadow implies a forthcoming demise. Despite the gloomy environment on Hitogashima Island, Shinpei perseveres, determined to fulfill Ushio's final wishes of safeguarding her sister, Mio.