Siblings Utsutsu and Yume Hasegawa, abandoned by their abusive parents, follow alluring red butterflies into a nightmarish ordeal. Unbeknownst to them, these crimson insects herald the emergence of a cannibalistic virus, Pupa, triggering a horrific transformation in Yume. As she becomes a ravenous monster, Utsutsu gains extraordinary regenerative abilities. Determined to uphold a longstanding promise, Utsutsu vows to stand by his sister through her monstrous plight, battling to contain her voracious appetite and shield the world from her destructive urges. Pupa portrays the poignant tale of a devoted brother's unwavering efforts to rescue his sibling while safeguarding humanity from her uncontrollable cravings.