Shuuichi Kagaya appears to be a typical high‑school sophomore, yet occasionally he morphs into a monster. He has no idea how or why these powers surfaced; all he wants is that nobody discovers them. One evening, he comes across a blazing building with a girl trapped inside. Choosing to rescue her, he transforms and lifts her out of danger, only to drop his phone in the process.
The following day, Claire Aoki—the girl he saved—tracks him down and accuses him of being a monster. She even pushes him off the school roof to prove her theory after he denies it. Desperate, he turns into a monster again, and Claire takes a photo to blackmail him into revealing everything she knows about monsters—information that turns out to be surprisingly scarce.
Claire, however, has her own secret: she"s been hunting for her sister, who also became a monster. She asks Shuuichi to help locate her, but they"re not the only ones seeking answers.
