There are twelve-hundred million episodes throughout the franchise featuring aliens attempting to take over the bodies of the regular crew. This is an early one. In that regard, it's so completely average that you hardly have to watch this one in particular to know and/or care about the proceedings. However, it's a precedent, part of a Star Trek trope, whatever way of saying it that makes you think it's worth watching in some way, at least in that regard.
It's also a Scotty episode.
|via Daily Drew|
But this one's a romance. Unlike "Wolf in the Fold," this is a good thing for the chief engineer. It's about as shocking to have a romance feature someone other than Kirk as it is to discover you're watching a Scotty episode, so for combining the two, it should be considered a landmark of some kind.
The last and perhaps most significant contribution of "Zetar" (the title evokes classic science fiction if not Star Trek in general, one of those titles that leaves no real impression of the story it represents) is the debut of Memory Alpha, the Federation library that was subsequently adopted by the fans, as you may have noticed a time or two.
In that regard, you wish, once again, that the series were less pedantic in the types of stories it was willing to do, not merely yet another harrowing alien encounter, but a simple Starfleet visit to the library. Sounds crazy, I guess, but even adding in Scotty's romance, that could've sufficed, couldn't it?
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