A small group of explorers have just returned to Earth after a several year trip to an earthlike planet of Alpha Centauri. They find, to their dismay, that they, literally, can't live on Earth at all. Anyone who has been away for at least 3 full years contracts a murderous disease called Earth Allergy Syndrome, or EAS. Also, relativity has reared its head; the toddler daughter of one of the explorers is now a middle-aged grandmother. The explorers are forced to return to Alpha Centauri, to what is now called Genser's World (after one of their group who succumbed to EAS).
Back on Genser's World, the colonists find the de-evolved descendants of another spacefaring race, and their empathic symbionts. Think of the symbionts as flying creatures the size of a hawk, with the face of a cat, and they love to be petted and scratched. They form bonds with humans almost instantly, a bond that becomes impossible to break. The colonists also come into contact with two cyborg intelligences, and a computer intelligence which is about to evolve into something that could easily wipe out the humans.
Suddenly, all contact with Earth is lost. By this time, the intelligences have modified the colonist's ship so that a trip of several years duration back to Earth has been reduced to several weeks. A group of colonists returns to Earth, and finds a scene of total devastation. Just before "The End," an asteroid was hollowed out and turned into a colony ship. It was launched toward Genser's World, with over 500 people on board. Their intention is to turn Genser's World into a fascist utopia.
This is a first-rate planetary colonization story with lots of science. Engineers and scientists will enjoy this book; so will everyone else who likes good, interesting writing.