Marak, who is mad, is taken to the Ila at the desert world's city with all the other mad. She pulls him out of the others because he and his father had fought against her (and lost). She says the words the mad hear are coming from an area in the east -- a bad area of the planet -- and if he goes and comes back to tell her what's happening there, she'll take care of his mother and sister. (His father threw them out when Marak admitted he was mad.)
The mad go with a caravan and find out they all see the same stories and hear the same voices. We hear a lot of desert stories and problems. When they reach their destination, it's another tower that is more obviously a rocketship. Luz, who is there and has been sending the voices, because she put the "makers" -- DNA -- in them when they were born. They screwed up because she wanted them there before they were 30 because another alien group only gave her that much time before they throw rocks from the sky.
Marak and his new two wives and some from the caravan get back to the Ila and the city is surrounded by tents -- all the tribes and villages have come to her for help -- and he tells the Ila about Luz. They also tell her that everybody has to get to Luz's rocketship immediately or they'll die in the bombardment. She finally agrees to go.
Marak becomes the leader (and mediator) and they have another even much more difficult trek to Luz's rocketship. Lots of people die, but as many were alive as possible. The last chapter, which should have been an epilogue, gives a scene from three generations later (Marak and his friends get to live forever, or close) and how the people are living on the widely changed planet.
Now, if those are rocketships, why can't they communicate with each other directly instead of sending people back and forth? Why do they need to send out people to find out what the world looks like instead of putting up a satellite? Why is there no electricity and the Ila uses oil lamps?
The DNA part is that the Ila has infested the people on the planet with her makers, but Luz has sent out her makers that were more powerful than the Ila's and that's why many of them live through the last caravan.
It's hard to think that rocketships and "makers" are really SF in the middle of a desert story. The desert story is pretty good, but it's not SF.
The other book in the "Gene Wars Universe" is Forge of Heaven and is not directly related to this one. I'm going to start it tonight and if it's mostly a desert story, I probably will stop.