ヒトを食う異形の怪物と化した妹と、自らの肉体を餌として捧げる兄の、究極の兄弟愛を描くテレビアニメーション作品。原作は茂木清香による同名人気コミック。監督は「ローゼンメイデン」の望月智充、アニメーション制作も同作のスタジオディーンが担当。
両親の離婚後、二人きりで生きてきた長谷川現と夢の兄妹は、赤い蝶を目にした日を境に生活が一変する。妹の夢はヒトを食べずには生きていけない怪物となり、兄の現はどんな傷を負ってもすぐに回復する能力を身につけた。黒装束を身にまとった婦人マリアからの提案により、現は怪物への変化を抑制する薬を服用し、自らを妹の“生き餌”とすることを決意する。だが二人は謎の集団にとらえられてしまい…。
Abandoned by their abusive parents and with only each other to depend on, siblings Utsutsu and Yume Hasegawa find themselves led astray by beautiful red butterflies that have appeared in their world. Unbeknownst to them, these crimson winged heralds trumpet the beginning of a cannibalistic nightmare—a mysterious virus known as Pupa is about to hatch.
After succumbing to the full effects of Pupa, Yume undergoes a grotesque metamorphosis into a monstrous creature with an insatiable desire for flesh; Utsutsu, on the other hand, is only partially affected, gaining remarkable regenerative powers instead. Reaffirming the resolve to keep the promise he made to himself years ago, Utsutsu is willing to sacrifice everything in order to always be there for his precious little sister.
Pupa tells the story of a loving brother's desperate struggles to save his sister, while protecting the world from her uncontrollable hunger.
