Shuuichi Kagaya appears to be an ordinary high school student, yet he occasionally transforms into a monster. He is unaware of the origin or cause of his powers, only that he wishes to keep them hidden. One night, he discovers a burning building with a girl trapped inside. Choosing to rescue her, he shifts into his monstrous form and carries her to safety, though he inadvertently leaves his phone behind. The following day, the girl he rescued—Claire Aoki—locates him and challenges him regarding his monster identity. After Shuuichi denies her claims, she pushes him off the school roof to test her theory. In a desperate attempt to save himself, he transforms, and Claire takes a photo to blackmail him into revealing everything he knows about monsters, which, ironically, is not much. It turns out Claire harbors a secret of her own: she has been looking for her sister, who also turned into a monster. She recruits Shuuichi to help track her down, but they are not the only ones seeking answers.