Grieving mother Magos cuts out a piece of her deceased 11-year-old son Santiago’s lung. Acting on fierce maternal instinct and the dubious logic of an old folktale, she nurtures the lung until it gains sentience, growing into the carnivorous little Monstrilio she keeps hidden within the walls of her family’s decaying Mexico City estate. Eventually, Monstrilio begins to resemble the Santiago he once was, but his innate impulses --- though curbed by his biological and chosen family’s communal care --- threaten to destroy this fragile second chance at life.