Growing up with an abusive father, Utsutsu is highly protective of his younger sister Yume. One day, Yume sees a red butterfly and contracts the strange Pupa virus. She begins to sprout grotesque wings and to indiscriminately attack both animals and humans. Hoping to find a way to stop the advance of the virus, Utsutsu seeks out Maria, a researcher who knows the secret of Pupa. He decides to sacrifice his own body as live bait for the sake of saving Yume.