That was the episode with the salt vampire. It really freaked me out as a kid.

RadioActive wrote:

Never heard this story before, but apparently when NBC decided to air the very first Star Trek in 1966, they picked the one episode the cast disliked the most. In a "who knew?" revelation, the network was afraid that some of the better sci fi plots they did would be too offputting to a general audience who had never seen a show like it before, so they picked the blandest and easiest one to relate to. 

The result was "Man Trap," a script that most of the cast, including William Shatner and Leonard Nimoy, thought was one of the weakest of all the scripts in Season

RadioActive wrote:

A letter-writing campaign (which I remember taking part in as a kid) helped save the show for one year, but despite the volume of response, NBC decided to cancel it anyway at the end of the next season. 

And here is that very article from Feb. 1968 I remember reading as a kid that prompted me to write that letter to NBC, the only time I've ever done anything like it. It was part of a school project to get students to "read the newspaper," an assignment that would scarcely exist today! 

It came with the headline: How To Save Star Trek: Keep Those Letters Coming

In the end, it worked - but only for another year.

Star Trek was doomed pretty much from the begining. Season One it was up against then #7 Bewitched. Season Two it had to contend with #3 Gomer Pyle. The letter writing campaign did get a 3rd season renewal. What did NBC do? Bury it Fridays at 10pm. The only other option would have been Mondays opposite Gunsmoke. NBC decided Laugh-in would be a better option against Marshall Dillon. Officially NBC received 114,667 letters between December 1967 and March 1968. For those interested I found much of this information in a rabbithole site entitled  Television Obscurities: Exploring Forgotten TV From The 1920's to Today.