After enduring years of traumatic abuse from their cruel father, siblings Utsutsu and Yume Hasegawa are ultimately left alone. They continue their lives with Utsutsu doing everything he can to take care of his little sister.

However, their short-lived period of peace comes to an end when Yume suddenly transforms into a horrific abomination and goes on a rampage, killing people and consuming their flesh. It turns out that Yume has contracted a peculiar and obscure virus named "Pupa." Although he is shocked at the sight of his sister's monstrous appearance, it seems that Utsutsu also carries this strange disease. He, however, is only partially affected, with the virus giving him regenerative powers beyond that of a normal human being.

Even so, Utsutsu remains unyielding in his resolve to protect his sister, even if he has to sacrifice his own body for her sustenance. Nevertheless, all of their problems have only just begun, as they uncover the mysteries behind the horrors that afflict them.