Utsutsu and Yume Hasegawa, two siblings who suffered years of traumatic abuse from their father, are left to fend for themselves. Utsutsu becomes the caretaker for his younger sister, but their fragile peace is disrupted when Yume transforms into a monstrous creature and begins killing and eating people. It is revealed that Yume has contracted a rare virus called "Pupa." Shockingly, Utsutsu also carries the virus but only shows partial symptoms, gaining incredible regenerative abilities.
Despite his sister's terrifying appearance, Utsutsu is determined to protect her at all costs, even if it means sacrificing his own life. But their troubles are only just beginning as they unravel the mysteries behind their horrific conditions.
