The Ringworld Engineers is a science fiction novel by Larry Niven, the second in the Ringworld trilogy. It was first published in 1980. This is in my opinion the best of the three books.

The set-up

"It's easy to show that the Ringworld is unstable. Stable along the axis, but unstable in the plane. There must have been something to keep the sun on the axis"
"But it's off center now!"
"Whatever it was stopped working"
Chmeee clawed at the invisible floor "But then they must die! Billions of them, tens of billions - trillions?"

"Why is the Ringworld unstable? Is it not in orbit?"
"No, of course not. It has to be rigid. That terrific spin would pull it rigid. If you nudge the Ringworld off center it'll fall further off center."

The Hindmost said "At one time we thought we might build our own Ringworld. The instability is too great. Even a strong solar flare would exert enough pressure to throw it off balance. Five years later it would grind against its sun."

The puppeteer trotted into view. "One year and five months from now, the Ringworld will graze its sun. I expect it will disintegrate then. Given their rotational velocity, the fragments would all recede into interstellar space"
- Ringworld engineers

Plot summary

NB: Spoilers, nothing but spoilers ahead.

Twenty-three years after the events of Ringworld, Louis Wu is living in hiding on a colony world, afraid of the earth government. When he and Halrloprillalar (Pril) returned to earth, Pril was taken into custody by the earth government - she was neither quite human nor alien. The whole thing was made top secret. Louis has also become an addict of electrical brain stimulation - a wirehead.

We rapidly meet Hindmost, a puppeteer, and former ruler of the puppeteer civilisation. We learn that decapitation and parenthood have cured Nessus, Hindmost's mate, but that now Hindmost's political party are out of power, in favour of a more isolationist faction. Hindmost himself seems to be afflicted with a different kind of madness, akin to megalomania. He wished to regain ascendancy by returning to the Ringworld.

The Kzin Speaker-to-animals (also a parent now, and who has gained the name Chmeee and has high status among the Kzinti) and Louis are lured into his ship and taken to Ringworld. We learn that Pril, who would have been the fourth member of the crew, is dead 18 years ago - she did not adjust to earthly longevity drugs.

The Ringworld is drifting of center and will not last another year. The puppeteer desires the transmutation technology that was obviously used to construct the Ringworld, as this will swing the balance of power back in favour of his party.

Hindmost controls Louis via his addiction, and Chmeee has been given a new Kzin-longevity drug that is making him young and in the process erasing his distinctive scars. He will need Louis's word and a sample of the drug to confirm his identity, and regain his property when he returns.

Once there, Louis and Chmeee inspect spaceship dock on the rim wall, but a suspicion mounts that Pril was lying when she said that her people built the Ringworld. Her descriptions of transmutation technology turn out to be inaccurate.

Louis and Chmeee land on the Ringworld surface. They are now out of Hindmost's control, though they still depend on him for a way off the Ringworld. Evidence of speciation among the sapient (i.e. specialised hominids) causes Louis to put the age of the Ringworld at a minimum of 100 000 years. Louis breaks his addiction.

They learn of Rishathra (sex between species who cannot breed together) and meet several species: Herbivorous, aquatic, carnivorous and nocturnal hominids. Vampires, who have devolved away from sapience, but evolved pheromonal lures to catch their prey. They decide to look for the "control center" of the Ringworld.

Via one of Hindmost's trade goods (superconducting cloth and wire that is immune to the germ that caused the fall of the cites, over a thousand years ago) it becomes apparent that the puppeteers cause the fall of the cities, with the intention of moving in to trade a replacement. But a change of government to the cautious faction caused them to back out of the plan.

Along the way Louis does good deeds, picks up travellers, and vows to save the Ringworld if he can. After they pass over 1:1 scale maps of several worlds (yep: life-size maps), Chmeee takes the lander and strikes out to conquer the map of Kzin (containing actual Kzinti), with some success. Louis destroys Hindmost's ship's hyperdrive, in order to commit him to saving the Ringworld. Louis and Chmeee come to the conclusion that the Ringworld must have been built by Pak protectors.

They find that the Ringworld had attitude jets to keep it centred, but that Pril's people had removed them and used them for starship engines, without any ill effects in the short term of a couple of thousand years. It seems that someone is busy remounting a few of these ramjets, but only one in twenty remain - not enough to save the Ringworld.

They locate the control room, under the map of Mars, and find Teela Brown there, but she is now transformed into a Pak protector. Her instincts and rational mind are in dire conflict- she knows how to save the Ringworld, but cannot bring herself to do it, as it will kill 5% of the Ringworld's population. She will not tell them how, but is leaving them sufficient clues for them to work it out.

Hindmost's ship is embedded in Lava under the map of Mars - safe, but immobile.

Louis and Chmeee fight Teela. She is fighting to lose, and they vanquish her with difficulty, and put the plan that she hinted at into action: They cause the sun to flare over the sections with attitude jets (isn't that a rapidly moving target?), giving the population under them a lethal dose of radiation, but giving enough fuel to the ramjets to right the Ringworld.

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