Following the death of his parents, Shinpei Ajiro lived with the Kofune family and their two daughters, Mio and Ushio. Although he later left his home island to pursue his education in Tokyo, Shinpei returns after Ushio tragically drowns while attempting to rescue a young girl. At the funeral, his best friend reveals bruises found around Ushio's neck, raising doubts about the cause of her death. Suspecting murder, Shinpei reexamines recent events, but strange incidents continue to unfold. Disappearances and other unexplainable occurrences lead Mio to recall an old folktale about entities known as "Shadows," which may not be entirely fictional. Allegedly, encountering one's Shadow predicts the person's impending death. Confronting the dark side of Hitogashima Island, Shinpei fights against his grim fate to fulfill Ushio's final wish: to protect Mio.