Science fiction is the genre of wild ideas, and this is a major novel of ideas from a great fantasy writer of, er, ideas. I'm not inclined to go explaining the plot at all. The cover blurb is a good precis without giving away any of the major developments, and part of the joy of this book is the process of working through the various ingenious conceits, so as well as a great story, cool world-building and excellent writing there's the intellectual thrill of seeing a set of arguments made and followed through to conclusions that are logical but unexpected That's what I fell in love with when I first read Isaac Asimov, and that's what I love best about science fiction now. A future classic.