It's tough to make a case for "Turnabout Intruder" being anything other than an exceptionally weak outing for "Star Trek: The Original Series." However, one might argue that it's not the worst episode the series — or the franchise in general — has ever produced, and some aggregators of fan reception would favor that idea.
As it stands on IMDb, "Turnabout Intruder" actually isn't the lowest-rated episode of "Star Trek: The Original Series." Instead, that dubious honor falls to Season 3, Episode 4, titled "And the Children Shall Lead," which has a strongly mixed score of 5.2 from user reviews. The episode sees the Enterprise crew cross paths with a group of young children seemingly oblivious to the fact that the adults in their community have died. While that mysterious premise may sound promising, fans have a general distaste for the direction of the episode. Other episodes, such as "Spock's Brain," have also been argued by some to be the show's worst outing.
All in all, whether "Turnabout Intruder" stands as the worst "Star Trek" episode of all time is ultimately a subjective matter that comes down to individual viewer preference. However, it certainly hasn't curried the "Star Trek" fanbase's favor in the way that many other installments of the sci-fi franchise have, and that isn't likely to change in the future.
