It is set in the solar system of a neutron star called Voy, where gravitational forces have drawn off the atmosphere of Voy's nearest planet into an enormous gas torus orbiting the star directly. This torus, the "Smoke Ring", is the only habitable part of the system, and is home to various native life forms, including fish that swim in free-floating pools of water, and the enormous, kilometers-long Integral Trees. Several centuries before the time of the story, an Earth colony came to the system, and its descendants now live in small tribes among the branches of the Trees.
The story follows a small group from Quinn Tribe who are sent up the trunk of their Tree in an expedition and become the only survivors when a disaster destroys their tribe's home. While searching for a new home, they visit other parts of the Smoke Ring and make discoveries about the history of the people who live there.
The story continues in The Smoke Ring (1987). Niven planned and partly wrote a third novel in the series with the working title Ghost Ships, but he abandoned it in the mid 1990s.
These novels contain examples of:
- Alien Sky: The "planet" is the sky. The Ring is a (mostly) gas torus from a supermassive gas giant in close orbit around a neutron star, which is a binary with a yellow dwarf.
- Amazon Brigade: The Triune Squads are made up of women who refuse to marry, women who love women, or those who are "women" by courtesy only. They are sent to patrol the Trunk, a hazardous and seemingly pointless duty, to make up for not doing their "real" duty to the tribe by providing children.
- Floating Water: "Ponds" are spheres of water floating in midair, due to the near-complete lack of gravity.
- Hold Your Hippogriffs: Several characters use the expression "feed the tree," which means, "The words you are saying are a commonly used form of natural fertilizer." In other words, bullshit.
- Improvised Microgravity Maneuvering: The tree-dwellers occasionally use high pressure "spitter" seedpods (that shoot seeds out once they are broken open on one end) as rocket motors.
- Life in Zero G: The Smoke Ring is a torus of gas around a star, and exists in an entirely zero-G environment. Its natives have adapted to this environment in a number of ways; they're very tall and thin compared to Earth humans, and have prehensile feet. The Smoke Ring's native life is further adapted for its conditions; notably, all local animals have trilateral symmetry, allowing them to see in a full ring around themselves, while the local humans retain the bilateral symmetry and forward-facing eyes of their terrestrial ancestors.
- Lightworlder: The human inhabitants of Integral Trees are somewhat taller and slimmer than Earth people, but they are strong, tough Heavyworlders compared to people from the rest of the Smoke Ring. The tidal forces create rotation in the trees, which provides at least a little simulated gravity, but everyone else grows up in zero-G.
- Lost Colony: The people inhabiting the Smoke Ring have lost contact with Earth, and most of the technology they brought with them.
- Mama's Baby, Papa's Maybe: After one female character is used as a Sex Slave, her husband decides that, no matter who the child looks like, the baby will be his child.
- Metal-Poor Planet: There is effectively no metal whatsoever in the Gas Torus; as such, all materials are made from local wood from the Integral Trees. What little metal there is has been recycled endlessly from what the colonists brought with them or is found in what are essentially meteorites.
- Sapient Ship: A sapient ship with an AI based on a recording of a now deceased checker, much like Peersa the Checker (a character in A World Out of Time) is a character.
- The Sky Is an Ocean: The novels are set in a thin, orbiting band of breathable air; the sky literally is an ocean, since there is no actual planet one can set foot on.
- Space People: The descendants of the stranded astronauts live in a cloud surrounding a star. They are extremely tall (around 3m) and slim and have elongated toes, as well as a high infant mortality rate due to the lack of gravity. One character, often referred to as a "dwarf", actually has an Earth-normal build; he's described as "monstrously strong" and is the only person who can wear one of the original spacesuits.
- Space Whale: The Moby, a whalish giant creature that lives within the Smoke Ring.
- World in the Sky: An unusual, real-physics variation. The smoke ring is a toroidal cloud of gas and matter which orbits a very low-output neutron star, which in turn orbits a sun-like star in a binary configuration. It includes a ring of breathable atmosphere, in which reside a number of flying plants and animals, including some humans who've "gone native".
- World Shapes: The "Smoke Ring" is a 'world' which is just a breathable ring of gas around a neutron star.
