Alpha Cas

Back in the twenty-second century, Earth was dying. World population was more than twenty billion, the global environment was in ruins, and living conditions were intolerable everywhere on the planet. Many people believed the only hope for humanity lay in colonizing space, and so giant ships were built to carry brave volunteers to settle the planets of nearby stars.
Many years later, two colonists from Alpha Centauri, Kellin Rhonden and Glendir Lywis, return to Earth in hopes of finding desperately needed medical technology. When they arrive, they discover a planet far different from the world their ancestors left behind. Although the problems of overpopulation, poverty, and environmental ruin have been eliminated, Earth has devolved into a world of two cultures separated by their opposing beliefs in evolution and the future of humankind.

No one is willing at first to believe Kellin and Glendir are from another planet, but when the leader of the the anti-evolutionary forces, Constance Dawkins, learns the truth, she sets out to capture the Centauris and destroy their spaceship. To complete their mission, Kellin and Glendir, with the aid of daring pro-evolution rebels, will have to escape the trap set by Dawkins, find the medical technology they need to save their planet, and free their ship.