Louis Wu is selected by the Pierson's Puppeteers for a secret mission. He and his companions: Nessus (an insane puppeteer), Speaker to Animals (a warloving Kzin) and Teela Brown (another human bred for luck), are to explore an unusual world where the world is a ring that surrounds a sun. Incredibly huge to hold a large population. 
 The Ringworld is not what they expected and after crash landing, they explore it to find a way off. 
 This was a very fun exploration book. With lots to think about and question as do our characters.