Surrounding a decaying neutron star is a torus of breathable air, the Smoke Ring, wherein--in near-weightless conditions--float some unusual flora and fauna: gigantic trees shaped like mathematical integration signs; cubic-mile globules of water (""ponds""); globular jungles; creatures large and small; and. . . some tree-dwelling people, attenuated descendants of a space survey team who fled into the Smoke Ring half a millenium ago to escape ill-defined but apparently totalitarian Earth government. So the half-exciting, half-predictable plot here concerns the adventures of the semi-barbarous Quinn tribe--forced to migrate after their tree dies. They eventually make contact with the more technologically advanced, slave-taking inhabitants of London Tree, inheritors of an old but still workable spaceship's cargo-and-repair module. And perilous journeys ensue--plus battles, enslavement, revolt, and finally an inadvertent trip outside the Smoke Ring. . . where a cyborg-computer-spaceship Earth representative waits to reassert control over its erring charges. About-average work overall from the author of Ringworld: one-dimensional characters, some hard-to-swallow concepts--but inventive, carefully worked-out, vigorous, and well-paced.