Ringworld is a 1970 science fiction novel by Larry Niven, set in his Known Space universe and considered a classic of science fiction literature. Niven later added four sequels and four prequels. (The Fleet of Worlds series, co-written with Edward M. Lerner, provides the four prequels, as well as Fate of Worlds, the final sequel.) These books tie into numerous other books set in Known Space. Ringworld won the Nebula Award in 1970, as well as both the Hugo Award and Locus Award in 1971.
On planet Earth in 2850 AD, Louis Gridley Wu is celebrating his 200th birthday. Despite his age, Louis is in perfect physical condition (due to the longevity drug boosterspice). He has once again become bored with human society and is thinking about taking one of his periodic sabbaticals, alone in a spaceship far away from other people. He meets Nessus, a Pierson’s puppeteer, who offers him a mysterious job with almost no details. Intrigued, Louis eventually accepts. Speaker-to-Animals (Speaker), who is a Kzin, and Teela Brown, a young human woman who becomes Louis’ lover, also join the crew.
They first go to the puppeteer home world, where they learn that the expedition’s goal is to investigate the Ringworld to see if it poses any threat. The Ringworld is a gigantic artificial ring about one million miles (1,600,000 km) wide and approximately the diameter of Earth’s orbit (which makes it about 600,000,000 miles or 950,000,000 km in circumference), encircling a sunlike star. It rotates to provide artificial gravity 99.2% as strong as Earth’s from centrifugal force. The Ringworld has a habitable, flat inner surface (equivalent in area to approximately three million Earths), a breathable atmosphere and a temperature optimum for humans. Night is provided by an inner ring of shadow squares which are connected to each other by thin, ultra-strong wire. When the crew completes their mission, they will be given the starship in which they will travel to the Ringworld; it is far faster than any possessed by humans or Kzinti.
When they reach the vicinity of the Ringworld, they are unable to contact anyone, and their ship, the Lying Bastard, is disabled by the Ringworld’s automated meteoroid-defense system. The severely damaged vessel collides with a strand of shadow-square wire and crash-lands near a huge mountain. Many of the ship’s systems survive intact, including the faster-than-light hyperdrive, but the normal drive is destroyed, leaving them unable to launch back into space to use the hyperdrive. They set out to find a way to get home.
Using their flycycles (similar to antigravity motorcycles), they try to reach the rim of the ring, where they hope to find some technology that will help them. It will take them months to cross the vast distance. When Teela develops “Plateau trance” (a kind of highway hypnosis), they are forced to land. On the ground, they encounter apparently human natives. The primitive natives, who live in the crumbling ruins of a once advanced city, think that the crew are the engineers who created the ring, whom they revere as gods. The crew is attacked when they commit what the natives consider blasphemy (the misuse of certain technologies).
They continue their journey, during which Nessus reveals some Puppeteer secrets: they have conducted experiments on both humans (breeding for luck via Birthright Lotteries: all of Teela’s ancestors for six generations were born from winning the lottery) and Kzinti (breeding for reduced aggression via the Man-Kzin wars, which the Kzinti always lost). Speaker’s outrage forces Nessus to flee and follow them from a safe distance.
In a floating building over the ruins of a city, they find a map of the Ringworld and videos of its past civilization.
While flying through a giant storm, caused by air escaping through a hole in the Ring floor due to a meteoroid impact, Teela becomes separated from the others. While Louis and Speaker search for her, their flycycles are caught by an automatic police trap designed to catch traffic offenders. They are trapped in the basement of a floating police station. Nessus enters the station to try to help them.
In the station, they meet Halrloprillalar Hotrufan (“Prill”), a former crew member of a spaceship used for trade between the Ringworld and other inhabited worlds. When her ship returned to the Ringworld the last time, they found that civilization had collapsed. The crew managed to enter the Ringworld, but some of them were killed and others suffered brain damage when the device that let them pass through the Ringworld floor failed. From her account, Louis surmises that a mold was brought back from one of the other planets by a spaceship like Prill’s; it broke down the superconductors vital to the Ringworld civilization, dooming it.
Teela reaches the police station, accompanied by her new lover, a native “hero” called Seeker who helped her survive. Based on an insight gained from studying an ancient Ringworld map, Louis comes up with a plan to get home. Teela, however, chooses to remain on the Ringworld with Seeker. Louis, formerly skeptical about breeding for luck, now wonders if the entire mission was caused by Teela’s luck, to unite her with her true love and help her mature.
The party travels back to their crashed ship in the floating police station. They carefully collect one end of the shadow-square wire that was snapped when the ship crashed, dragging the wire behind them. Louis threads the wire through the ship to tether it to the police station. He then takes the police station up to the summit of “Fist-of-God”, the enormous mountain near their crash site. The mountain had not appeared on the Ringworld map, leading Louis to conclude that it is in fact the result of a meteoroid impact with the underside of the ring, which pushed the “mountain” up from the ring’s floor and broke through. The top of the mountain, above the atmosphere, is therefore just a hole in the Ringworld floor. Louis drives the police station over the edge, dragging the Lying Bastard along with it. The Ringworld spins very quickly, so once the ship drops through the hole and clears the ring, they can use the ship’s hyperdrive to get home. The book concludes with Louis and Speaker discussing returning to the Ringworld.