The book follows the adventures of Freya Nakamichi, a sex 'droid designed to please her human masters. Unfortunately for Freya, human beings have been extinct for two centuries or so, leaving us with a character with no idea what to do with her life. Most robots designed to serve human beings were cute, anime-like designs for household use, but Freya's shaped like the real deal, a tall ogre out of place in a world of short bishi and chibi designs. Depressed and despondent, she takes a job as a courier, winds up in all kinds of trouble, and ends up careering around the solar system, gets possessed by the spirit of her dead sisters, and eventually comes face-to-face with the biggest dream and fear every robot has: meeting a real live human being.
Unfortunately, this book falls off the end of the world toward the last chapters. Up until the info-dump where Freya reveals the true nature of robot devotion to human beings, a ham-handed scene if ever there was one (although fortunately the worst of it is ob skene), I was convinced that Charlie was going somewhere interesting with the book. Charlie mentioned that the book is an homage to Robert Heinlein (and the final set piece of the book is set in Heinleingrad, Eris), and the end of the book is as unconvincing as the ending of Freya's namesake novel, Heinlein's Friday. At the end of Friday, you might recall, the titular character ends up marrying the guy who raped her at the beginning of the book ("it was just business") and running away to some far away stellar colony, leaving Earth to collapse under its own corruption. The ending of Saturn's Children ends with a very similar, and even more serious problem, left unresolved: robots who are honest with themselves about their origins are terrified that H. Sapiens might someday re-emerge and assert their right to rule, disrupting the free will of the machines. It's presented as the central conflict of the main character, emerging throughout the book, growing in intensity as Freya gets closer and closer to meeting an authentic H. sap, only to be ignored in the final two chapters in favor of pyrotechnics and "aww, aren't they sweet" moments.
Charlie's ability to create engaging, intense, and intensely clever tight spots from which his heroine must escape, often with that classic transition, frying pan, fire, is here in all its glory. He does a great job of cranking up both the threat and the resolution, over and over again, while weaving a Sol-spanning conspiracy that should ultimately leave you breathless. Charlie knows how to dress the stage and then set the furniture ablaze a la Jack Bickham, and his technical hard SF knowledge is second to none. But if Saturn's Children is a Heinlein pastiche and an Asimov homage, it's also unfortunately got something else: A Neal Stephenson ending.