A mostly different batch of folks return to World, one being a really annoying very smart physicist, and they excavate the artifact in the holy mountains. It turns out to have settings that are alternately shield or weapon and they decide to take it back to Sol System for safety. When they leave, two of the original scientists stay on World to help the natives deal with the loss of shared-reality (moving back to feudal level). Up on the ship, secret from almost everybody, is one of the enemy Fallers and a Sensitive (gene-mod) is trying to talk to him. The final ship scenes are the most interesting part of the book, and the military matters will be familiar and funny to anybody with military knowledge or experience.
I don't think you can read this book without the first, but it's not a bad sequel and it's a much better book. I'm going to start Probability Space tonight.