There's some plot with the Immortals down on the planet, but most of it is on a station above the world that has two types of humans plus the aliens.
There are "taps" -- people who can hear someone else's voice -- who, in three shifts, are constantly with the Immortals. They can give them information from satellites so they know what's happening with the weather and the continuing changes to the world. Basically, they become help to the Immortals. The daytime tap for Marak, the hero of the other book, is named Procyon and Marak picked him personally.
An Earther ship lands at the station out of cycle and turns out to have an ambassador, in a quarantine carrier because they're afraid of the Outsiders, who wants to talk to Procyon. In the beginning, this doesn't seem to be such a problem, but when he does visit the ambassador, someone launches a rocket at them.
The various governments, layers of the station, spies, and taps are all full of intrigue, back-stabbing, and disaster. Then the alien steps in.
I really liked this book and highly recommend it. You don't have to read the first one.