Tasuku Tsuchiya, a high school student, has a painful past: his younger sister, Akari, was taken by a "monster's arm" that appeared from the void. Then, a strange transfer student appears before Tasuku, her right arm wrapped in peculiar bandages. Her name is Mikoto Amane. Mikoto keeps her distance from her classmates and doesn't try to befriend them. One afternoon after school, Tasuku, who happened to be following Mikoto, witnesses something...