In her raw, desperate grief, Magos cuts out a piece of Santiago's lung—a preserved lobe that she keeps in a jar. She feeds it, nurtures it, and eventually, it begins to grow. It becomes a creature she names Monstrilio. The novel then follows this being over several years, told in four parts by different narrators: Magos, her best friend Lena, a young man named Oliver, and finally, Monstrilio itself.
As the creature grows, it begins to resemble Santiago physically—but with sharp fangs and violent, animalistic instincts that it struggles to control.