The newly liberated humans of the Fleet of Worlds and Sigmund Ausfaller, who had been transported by the Puppeteers from Earth to the Fleet, must contend not only with the sly Puppeteers but also the threat of the Pak, a very smart and utterly ruthless species who are fleeing the exploding galactic core in an armada of ships at near light speed. The Pak are headed towards the Fleet of Worlds, having destroyed entire planets in their wake.

Since fleeing the supernova chain reaction at the galactic core, the cowardly Puppeteers of the Fleet of Worlds have - just barely - survived one crisis after another: the rebellion of their human slaves, the relentless questing of the species of Known Space, the spectacular rise of the starfishlike Gw'oth, the onslaught of the genocidal Pak. Now fresh disaster looms, as though past crises have returned and converged. Who can possibly save the Fleet this time?

Fleet of Worlds takes a closer look at Human-Puppeteer (Citizen) relations and the events leading up to Niven's first Ringworld novel. Kirsten Quinn-Kovacks is among the best and brightest of her people. She gratefully serves the gentle race that rescued her ancestors from a dying starship, gave them a world, and nurtures them still. If only the Citizens knew where Kirsten's people came from.

For decades, the spacefaring species of known space have battled over the largest artifact - and grandest prize - in the galaxy: the all-but-limitless resources and technology of the Ringworld. Now, without warning, the Ringworld has vanished, leaving behind three rival war fleets. If the fallen civilization of the Ringworld can no longer be despoiled of its secrets, the puppeteers will be forced to surrender theirs - everyone knows that they are cowards. But the crises converging upon the trillion puppeteers of the Fleet of Worlds go far beyond even the onrushing armadas.

The Ringworld is a landmark engineering achievement, a flat band 3-million-times the surface area of Earth, encircling a distant star. Home to trillions of inhabitants, not all of which are human, and host to amazing technological wonders, the Ringworld is unique in all of the universe.