"Return of the Archons" is the original Prime Directive episode, somewhat comparable to the earlier "Miri," in which the crew discovers a civilization that has been considerably warped bu a given set of circumstances, a trope that would be repeated throughout the series and indeed franchise.
As with some later episodes ("The Tholian Web," "Friendship One" from Voyager), the impetus for the story stems from the recovery of a missing Starfleet vessel. Also like later episodes (such as "The Communicator" from Enterprise, plus from this one, such as "Patterns of Force"), Starfleet has inadvertently affected the course of evolution on a planet, although in this case (as with several other episodes, of course!) it certainly doesn't help that a meddling machine has swooped in to "perfect" the resulting society.
All of which means that our crew finds itself unwitting new victims to the status quo, and must grapple with how to get themselves out of the situation, and wondering for the first time if they have a right to interfere. As such, the Prime Directive, which forbids Starfleet officers from knowingly affecting the course of change in an alien culture.
Overall, "Return of the Archons" is itself an episode you can forget easily enough, enough to celebrate for fans just watching the series through, but its effect is pretty strong all the same.
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