Earth has nearly destroyed itself -- we're never told the details -- and a small group of Quaker colonists has made its way to a planet they call Pennterra. It's a beautiful planet, and while humans can't eat the local flora or fauna, their own plants and animals grow well there.
It's a difficult life, though, in large part because of the restrictions placed on them by the native sentients, the hrossa. The hrossa insist that the humans must limit themselves to one small valley, that they may not use any heavy machinery, and that they must not allow their population to increase significantly above their original numbers.
A new group of colonists has just arrived, though -- they left before hearing the Quakers' "Don't send anyone else" transmissions -- and this group is far less willing to put up with the hrossan restrictions. They certainly don't believe the warning that violations of these rules will lead to their destruction -- not through any act of violence on the part of the hrossa, but destruction caused by the planet itself.
It's a promising setup, with lots of opportunity for conflict and misunderstanding among the planet's three communities. And Moffett makes the most of it, creating a society and ecology that are wildly different from our own, and showing us how human and hrossa change one another.
The prudish should be warned that sex and sexuality play a large part in the plot; one of our characters is the first human to go through puberty on Pennterra, which turns out to have some unusual psychological implications. While Moffett isn't sexually graphic, there are some goings-on that might shock a few even today, and must certainly have raised eyebrows 25 years ago. But they're presented as reasonable, understandable developments, given the background she's established, and none of it is meant as mere titillation.
Pennterra is a smart and provocative novel that offers a lot to think about, but never becomes so didactic or idea-heavy that it ceases to entertain.