Fifteen-year-old Christie and his older stepbrother, Danny, travel to the home and mysterious laboratory of the elder boy's scientist mother, where they learn a shocking truth about the nature of her experiments.
Christy 's souvenir stone from the yeti's cave proves to be fatal for many of the inhabitants of Fourth World, leaving among the survivors a pair of genetically-altered twins whose eventual descendants face an uncertain future in a stone age world of nuclear devastation and disease.
