It's the debut of the Klingons! Need I say more?
Well, okay. The whole dynamic for the rest of the series is there right from the start, which is only natural, given that this is coming (finally!) near the end of the first season, so most of what Star Trek would settle into being had been established. That means that the Klingons who show up are exactly the kind of Klingons who would show up in other episodes.
Unlike the Romulans who preceded them, the Klingons are presented with ongoing potential, which is probably why it was so much easier to explore them. The Romulans were certainly interesting in "Balance of Terror," but their Cold War analogy was limited to the war itself, which to that point in history had become more and more remote a possibility (besides the proxy Vietnam conflict). No, if American viewers were to see alien versions of the Russians in space, they would more likely succeed as competitors. And that's exactly what the Klingons were.
Between Starfleet and the Klingons for this episode stood the Organians, the latest in a long line of omnipotent, meddling aliens, who impose peace between the foes (it didn't stick). That particular aspect of "Errand" has grown less significant over time, the more nuanced the Star Trek landscape has become.
But Kor stuck around. Oh, did he! One final counterpoint with the Romulans, whose figurehead had gone unnamed in his duel with Kirk, the first Klingon we meet is named, and three decades later becomes all the more important to franchise lore thanks to a series of appearances in Deep Space Nine (all while being portrayed by the same actor, John Colicos).
That alone makes this episode easy to revisit, even if the Organian elements have failed to age as gracefully. The link Kor makes between eras makes "Errand of Mercy" comparable to "Space Seed." Arguably it's far more enjoyable to see the young Kor in action than the young Khan (unless we're talking Into Darkness). Often fans of the series claim that it managed to buck the trend of the later ones by having its greatest material in its first season (the other tended to struggle with that). I don't think that's necessarily true (the second season is much more consistent), but "Errand" is the kind of episode you can point to, beyond "Balance" or "City on the Edge of Forever," where it's clear that no matter the growing pains this was a series that really had managed early on to find its footing, and the lasting influence of the Klingons is a testament to that.
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