Pierson's puppeteers, strange, three-legged, two-headed aliens, have discovered an immense structure in a hitherto unexplored part of the universe.Frightened of meeting the builders of such a structure, the puppeteers set about assembling a team consisting of two humans, a puppeteer and a kzin, an alien not unlike an eight-foot-tall, red-furred cat, to explore it.The artefact is a vast circular ribbon of matter, some 180 million miles across, with a sun at its centre - the Ringworld.But the expedition goes disastrously wrong when the ship crashlands and its motley crew faces a trek across thousands of miles of the Ringworld's surface.