The third title in the Missing Link series again uses a split narrative to explore what it means to be human. In the near future, the journal of teenaged Christie recounts the increasingly deteriorating situation at Fourth World. The isolated family of “mad scientists,” talking animals and animal-human hybrids encounters a bizarrely alien organism with an agenda of its own. In the parallel story set some unknown time in the future, Nessa, a young Cat, has fled the violence between her own community and the rival tribe of Dogs. Reluctantly teaming up with a Dog named Farral, and accompanied by various talking animals, she is eventually forced to accept the truth about Cats, Dogs and the secret that binds them. The science here is ludicrous, and the sociology preposterous; the message of tolerance is laudable, if heavy-handed. The Fourth World segments, with their plethora of motley characters, off-kilter setting and references to earlier events, would appear unintelligible to those unfamiliar with the earlier volumes. The far-future plot stands on its own, and Nessa and Farral are engaging, believably complex protagonists, and their animal friends are likable and distinct. The final revelation tying the two story lines together is predictable, but not unsatisfying. For fans of the series only. (Science fiction. 12+)