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 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.

Covert agent Sigmund Ausfaller is Earth's secret weapon, humanity's best defense against all conspiracies, real and imaginary, and all foes both human and alien. Who better than a brilliant paranoid to expose the devious plots of others?

But Sigmund may finally have met his match in Nessus, representative of the secretive Puppeteers, the elder race who wield vastly superior technologies.

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.

It’s been 20 years since the quixotic and worldsweary Louis Wu discovered the Ringworld. Now he and SpeakertoAnimals are going back, captives of the Hindmost, a deposed puppeteer leader. With Louis’ help, the Hindmost intends to regain his status by bringing back such extraordinary treasures from the Ringworld that his fellow puppeteers will have to be impressed. But when they arrive, Louis discovers that the Ringworld is no longer stable - and will destroy itself within months. To survive, he must locate the control center of the legendary engineers who built the planet.