Twelve-year-old narrator David Ennis and his twin sister, Julie, are gradually drawn into an otherwordly web surrounding their heretofore perfectly ""normal"" 77-year-old aunt. The clues mount up--she's never seen eating or drinking; she's the only one able to revive the local tycoon from his strange daze; she even cracks a joke about ""pod people""--but David and Julie are already inextricably involved before they realize the truth: Aunt Margaret is a pod person. In a science-fiction plot mixing elements from Invasion of the Body Snatchers, Star Trek, and The Day the Earth Stood Still, David and Julie are heroes in helping Margaret to save the local Humane Society founder. A well-written tale with emotional dimensions, by the author of other sf novels, including several in the ""Star Trek"" series. The flow pace here, however, may lose the intended audience: not until almost halfway through the book do events shift fully from mundane small-town life to the hair-raisingly weird--with David literally getting into another person's skin.