A boy, Utsutsu, and his little sister, Yume, are siblings who find themselves all alone. After Yume's body undergoes a strange metamorphosis-into a creature that eats human, Utsutsu struggles to find a way to restore her.
The "life-and-death sibling" story follows Utsutsu and Yume Hasegawa, a boy and his little sister who find themselves all alone. One day, Yume sees a mysterious red butterfly and her body undergoes a strange metamorphosis—into a creature that eats humans. Utsutsu struggles to find a way to restore his sister.
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.
