Utsutsu and Yume Hasegawa, siblings who've grown up together, find themselves struggling with abandonment and abuse. Their lives become complicated when beautiful red butterflies start appearing, seemingly drawing them into a dark reality. A mysterious virus called Pupa is spreading, causing its victims to undergo a grotesque physical transformation. Utsutsu is only partially affected, gaining regenerative abilities, while Yume is completely changed, her desire for human flesh becoming uncontainable.
