This is the third book in this series by Charles Sheffield. It is set in a world where technology allows humans to change their bodies in myriad ways. Some of those ways are stable and legal, others are unstable, and still others are illegal. Some forms die quickly, while others can last for centuries. This technology is so integral to human society that it defines whether an infant is human or not. Using this technology and advances in space flight Humanity has colonized the entire solar system, all the way out to the Oort Cloud (Cloudland).
The story opens on a problem. The “Humanity Test” has failed in the independent colonies of the Kuiper Belt. A creature was born to human parents, but it lacks the ability to think. More importantly it lacks the ability to interact with the form change technology. The problem has been assigned to Sondra Dearborn, it is her job to find out why feral forms passed the humanity test. She approaches her distant relative, Bey Wolf, for help with the problem.
He refuses to help her, but the visit alone disturbs the forces that caused the humanity test to fail. As Sondra visits the Kuiper Belt and sees several unique cultures that rely on form change technology, Bey is decoyed to Mars in a needless effort to distract him from Sondra’s problem. Ultimately the effort backfires, as Bey finds the motive for breaking the humanity test in the Underworld on Mars.
Overall, this is a very enjoyable book. There are occasional references to the previous books, but not so much that reading them is required to enjoy this one. The dialogue is enjoyable, the characters interesting, and the premise thoroughly intriguing.