What is the main conflict in Ringworld by Larry Niven?

In Ringworld, 200-year-old human Louis Wu is recruited by a two-headed alien named Nessus to join him, a catlike warrior alien named Speaker, and the infinitely lucky human Teela Brown to explore an alien artifact. They find a Ringworld, a ribbon millions of miles long built around a distant sun. The civilization has fallen into savagery, though, and after crashing into the Ringworld, Louis must come up with a clever plan to get back to known space, hundreds of light years away.