The narrator relates how he has adapted to his immortality: he has found spouses - with whom he only stays for 30 years at a time, so as not to elicit suspicion - and has never pursued wealth or leadership positions, so as not to draw attention to himself.
The narrator makes fragmentary references to civilizational benchmarks he has witnessed, including interplanetary, interstellar, and even intergalactic travel, the planting of colonies on many worlds, and above all wars.
He makes a distinction between "minor" nuclear wars, which result in "mere" Dark Ages - a few centuries during which technologies are not forgotten, but the industrial base for them is lost - and "blowups," which follow the invention of much more destructive weapons, represent a complete loss of continuity, and require the rebuilding of civilization from scratch.
In something of a twist, the narrator names the six civilizations he has seen which have ended in blow-ups; the last of these are Mu and Atlantis, thereby overturning an unspoken implication that he is speaking from 180,000 years in the future.
Finally, the narrator speculates that - given the effacing powers of erosion and the loss of cultural continuity - the civilization into which he was born may not have been the first one, and that humanity may have been passing through these cycles since long before his birth.