Midworld is set in Foster's Commonwealth universe, and like other novels in this grouping such as Cachalot and Sentenced to Prism, it features a descriptively rich exotic environment where ecological balance is at the moral center of the novel. Ballantine Books provides the following summary:
Born was a child of the rain forest that covered Midworld...part of a primitive society that the peaceful tree planet had sustained for hundreds of years. He was wise in the ways of his world and he knew well the precarious natural balance that governed all things. Then one day the aliens came. Giants!
They knew nothing of the upper or lower Hell...and cared less. Born had risked his life to save them, to guide them through the myriad tangled boughs... past unseen, unsuspected dangers lurking in the underbrush.
As the days passed, Born realized his mistake. And as he had once hunted only to live...he knew now that he would be forced to live only to kill!