It was in the 70s. There were people flying through space on these world ships, giant ships with separate "pods" or sections that were self-contained.
They'd been on the ships so long they forgot they were on ships. But someone in one of the pods discovers the truth, and they set out to explore the other pods and tell them they're on this ship. And each pod developed its own different kind of civilization so half the story was how the people in the next pod ended up.