This story describes a civilization that used to be at a tremendously advanced level, well beyond our own in some distant past. It was encountered by space traders at about five hundred years before the story opens.
The initial premise is made out to be simple: A primitive tribe on the planet has rare plant that needed to cure an obscure disease elsewhere along the space trade routes. Traders trade what in the interplanetary economy must seem like an imbalanced deal: farm implements or pottery for rare medicine.
However, one particular occasion, the natives are not ready and usual with their prepared stocks, and seem to have somehow regressed to near caveman levels. The ship's captain is furious at his lost profits, but the ship team also carries a man who might be called an anthopologist, who sets out to figure the situation out.
It turns out that there is something of a wise-tragedy at work here. A formerly tremendously advanced race has just barely averted catastrophic wars, and makes a deep racial vow to never let that happen again. Their solution: to voluntarily give up their entire cultural heritage and return to what we would call Bronze-Age farming culture. But even that is not enough, for the beginnings of economic disparity are there, which sow the seeds of war.
So the real surprise is that they take regularly scheduled cultural regressions back to cavemen levels, on the theory that cavemen culture can't do much damage to itself in a warlike setting. It turns out that the same plant useful for medicine is also the social catalyst of this racial forgetting process.
Upon discovering that this culture, though at strange and terrible cost, the anthropologist prepares his recommendation to shut down all trade to the planet to avoid upsetting their cultural heritage.