The gigantic, cylindrical generation ship Vanguard, originally destined for "Far Centaurus", has been cruising without guidance through interstellar space ever since a mutiny long ago killed most of the officers.

Over time, the descendants of the surviving loyal crew have lapsed into a pre-technological culture that is marked by superstition, and have forgotten the purpose and nature of their ship.

The scientists ritualistically perform the tasks required to maintain the Ship, such as putting trash into its energy converter to generate power, and remain ignorant of their true functions.

He barely avoids getting eaten by the microcephalic dwarf Bobo and instead becomes the slave of Joe-Jim Gregory, the two-headed leader of a powerful mutie gang.

Joe is killed in the fighting, but Jim sacrifices himself to hold off their pursuers long enough for Hugh, Bill, Alan, and their wives to get to a highly automated lifeboat.

"[2] Algis Budrys said, "Many hands have worked at improving Heinlein's impeccable statement of this theme", with none succeeding until James White's The Watch Below.

[3] A paragraph at the start of the novel shows an excerpt from "The Romance of Modern Astrography," explaining that the ship was part of the "Proxima Centauri Expedition, sponsored by the Jordan Foundation in 2119" (A timeline produced by Heinlein to link different stories in his Future History places the launch of the Vanguard in the early 22nd century.

In Heinlein's later novel Time Enough for Love, the Vanguard is briefly mentioned as the sister ship of New Frontiers, which was commandeered by the Howard Families in the novel Methuselah's Children.

Its path is traced back, and the descendants of Hugh's people are found, flourishing as highly intelligent savages on a planet which scientists dub "Pitcairn Island".