No one intended to settle the nameless planet covered by seven levels of steaming tropical forest. A colony ship wound up orbiting it by mistake, giving the stranded passengers no other option except to land and try their best to survive. A few of them did that for long enough to have children. Now, generations later, their adapted descendants include a hunter named Born. He's a curious young man, so much so that his people call him mad. For what good is curiosity, in a place where seeking to see the sky involves going to a place called Upper Hell?
When two giants drop out of the sky, only Born cares enough about the mystery to climb down through the trees and rescue them from their crashed flitter. Although they know less than the smallest child about surviving in this perilous place, Born manages to keep them alive long enough to reach the relative safety of his Home Tree. His tribe thinks it's seen him for the last time when he sets out to guide the strangers to the "station" from which they say they came - no one ever traveled that far before. As Born learns more and more about these fellow humans whose thinking is so very unlike that of his own people, and as he discovers how they mean to use the awesome power they've brought to Midworld, the young hunter's curiosity turns to horror. These invaders have to be stopped from carrying out their plans. But how?
The world-building in this book amazes me. Parable and adventure tale, it works well on both levels; and it ends with a chilling twist that turns what had looked predictable squarely onto its head. Followers of Foster's Flinx books - the more recent ones, especially - will find some delicious foreshadowing in that twist.