23 years after his first visit to the Ringworld, Louis Wu is now addicted to Current - a drug with no hang over in which a small amount of electricity is delivered directly to the brain. Unlike most Current addicts, however, he is able to restrict his use of the drug to allow for time to exercise and sleep. This keeps him in good physical shape, though he does nothing but take Current every day. After 2 thugs break into his room, he decides to flee the planet (Canyon) he is on. He sees Speaker-To-Animals (now known as Chmee). Chmee and Louis are captured by the Hindmost - a Puppeteer and the mate of Nessus.

He has brought them together to return to the Ringworld in order to find a transmutation device (something Prill, the city builder who returned to Known Space at the end of Ringworld, claimed the Ringworld Engineers had used to create the Ringworld). This device will give the Hindmost the ability to reclaim political power in the Puppeteer worlds. He does not realize, however, that Prill was lying about the existence of this device.

Prill is meant to be the 4th member of the group, but they discover she was killed when taken into custody upon returning to Known Space. Chmee and Louis are put into stasis on the journey to the Ringworld. When they arrive, they are given a moment to discuss how they can try to escape from the Hindmost and return to earth. When they see the Ringworld, however, they realize it is off center and, in a year or so, will smash into its sun. They set off in search of a theoretical "Repair Center" where they convince the Hindmost the transmuter would exist. Louis is also determined to save the Ringworld to make up for the destruction of a shadow square, and the millions who were killed as a result during his first visit to the Ringworld.

They meet a species of carnivorous herdsmen who are humanoids. They help them defend their herd, and learn of rishathra - interspecies mating meant to show trust, seal treaties, etc. Next, Chmee and Louis use their landers speakers to pretend to be gods to a group of giant herbivores. They devise a scheme to destroy the sunflowers (deadly plants that kill by reflecting sunlight) nearby. They succeed and move on, towards a floating city.

Along the way, they come across a group of Machine Builders who are being attacked by silver haired humanoids. These are vampires that release a pheromone that renders humans unable to resist them. They kill in this manner. Louis is overcome with this desire, and Chmee, after killing the vampire, leaves out of disgust. Louis befriends Valavirgillin - a machine woman who was the only other survivor of the attack.

From Vala, and with the help of the Hindmost's probes, Louis learns that there are attitude jets on the Ringworld meant to fix the instability that is currently causing it to head towards the sun. Unfortunately the city builders of the past removed them for their ships. Only 21 remain, but they were added recently, meaning there is a repair crew working to fix the problem.

Louis now (correctly) postulates that the Ringworld was built by Pak protectors. Pak are humanoids that, at around age 40, must eat from the tree-of-life in order to evolve to become Protectors. If they don't, their teeth fall out, their hearts fail, joints swell, their skin wrinkles (effectively, human aging) and they die. They populated earth, but when the tree-of-life wouldn't grow there, they abandoned humanity. They are also no longer on the Ringworld, which explains the various forms of humanoid inhabitants that evolved after they died off.

In the floating city Louis uses superconductor cloth to fix water condensers and eventually buys his way into the Library. Once there, however, he must steal the tapes and a reader in order for the Hindmost to be able to read the information on them. During Louis's theft and escape, 2 librarians follow him - Kawaresksenjajok and Harkabeeparolyn. Kawa is a young boy, and Hark is a middle age woman. They reluctantly agree to help Louis when he explains how he is trying to save the Ringworld.

Louis realizes that the repair center is in the map of Mars located in the Great Ocean. First, however, he tracks down Chmee in the map of Kzin where he was injured after impregnating several females. Once at the map of Mars, they discover a floating city in Mons Olympus. They find an opening into the Repair Center, but are disabled and encased in molten rock. It is Teela who, since Louis left the Ringworld, found tree-of-life and became a Pak Protector. She has been attempting to repair the attitude jets, but she knows it won't be sufficient to save the Ringworld. Louis realizes she is using doublethink - her instincts won't let her do what she must to save the Ringworld, nor will it allow her to set Louis free to do it.

Louis, Chmee, and Hark escape from the ship and make their way to where the tree-of-life grows. When they find it, Teela attacks them. Louis and Hark both have their pressure suits breached. This causes Hark to go mad with desire for the plant. Louis, however, is able to resist thanks to his ability to conquer his Current addiction. He and Chmee are able to kill Teela. They then make their way deeper into the Repair Center, where they are able to use the Ringworld meteor defense system (which creates a magnetic field that creates aimed solar flares to destroy meteors) to overload the working attitude jets and get the Ringworld back on course. It causes 5% or 1.5 trillion humanoids on the Ringworld to die. Louis and Chmee are trapped on the Ringworld, but with how much space there is to investigate, Louis is content to adventure.