The Ringworld Engineers

by Larry Niven

The Hindmost, recently deposed leader of the Puppeteers, abducts the human Louis Wu (who has become a wirehead) and kzin Chmeee (previously known as "Speaker-to-Animals"). Both had been part of the Ringworld expedition in the first novel. The Hindmost hopes to acquire Ringworld technology, specifically matter transmutation, to help him regain his position.

Once they reach the Ringworld, Louis and Chmeee set forth to explore, while the Hindmost remains safely behind on their starship. Louis and Chmeee also secretly plot to try to overthrow the Hindmost so they can go home. In their travels, they meet a number of the hominid species that have evolved on the Ringworld. They also learn more about the full-scale "maps" of various known space worlds, including Earth, Mars and Kzin.

They discover that the Ringworld has become unstable and will collide with its star soon. The Ringworld's builders, revealed to be Pak Protectors, have long since died out, and the attitude jets they installed all around the rim to maintain the Ringworld's position were later dismounted to use as starship engines. Chmeee goes to the Kzin map for his own goals, while Louis tries to find some way to save the trillions of inhabitants.

It is on the Map of Mars that the reunited party (and two natives) finds the Ringworld control room Louis is seeking, located in a vast maze of rooms in the hollow space under the map. To create the rarefied atmosphere on Mars, the Map of Mars was built cavity. The control room contains living space and, among other things, the meteor defense system. The defense system uses the superconductor grid embedded in the Ringworld's scrith floor to manipulate the magnetic field of the Ringworld's sun to trigger stellar flares that power a titanic gas laser. (The first expedition to the Ringworld crashed after being hit by this laser.) They encounter Teela Brown, a human member of the first expedition who had chosen to remain behind twenty years earlier. She and her lover Seeker had, in the course of their travels, stumbled upon "Tree-of-Life" plants. The smell of the plant drove them to eat its roots; Teela was turned into a Pak Protector, with superhuman intelligence and strength, but Seeker died, being too old to undergo the transformation. As a Pak Protector, Teela has little control of her actions; her protective instincts force her to try to save all of the Ringworld inhabitants. This causes a dilemma. She knows of a way to save the Ringworld, but it would entail killing 5% of the people. This she cannot do. However, she manages to lure Louis and the others to where they can save the other 95%. Her instincts make her fight them, but she does so half-heartedly so that they can kill her.

Afterward, Louis figures out what to do. Teela had restored starship engines to their original purpose as attitude jets, but only had enough for 5% of the ring. Louis gets the meteor defense system to generate a massive stellar flare (normally used to power the laser weapon) to provide twenty times more fuel to the attitude jets to move the Ringworld back into position. However, the radiation from the flare is fatal to everything and everyone living on that section of the Ringworld.

Having earlier destroyed the hyperdrive to force the Hindmost to cooperate, Louis is stranded. He and the rest of his party look for someplace to settle down, while the Hindmost remains aboard the disabled (but very safe) starship to think things over.