The Enterprise encounters Nomad, a fusion of an Earth and alien probe that has a blended program to destroy all life forms that are imperfect.
If I was an executive producer in the late 60s, this is one of three "menacing computer" stories worth keeping, along with "Taste of Armageddon" and "The Ultimate Computer". The story here is not complex but frightening, and is hightened by the fact that this warped but all-powerful little probe moves among the crew and judges fitness on sight, almost arbitrarily. There are nice little references in the script, Nomad paying Spock the compliment of being "well-ordered" and Spock doubting Kirk "had it in him" to defeat Nomad during the conclusion. Nomad's increasing suspicion about his "creator" is suspenseful and Kirk's logic in causing Nomad to see its conflicted programming is even, well, almost logical in this episode.
Some of the flaws are irritating - Nomad could be just as scary without having all manners of mysterious beams and abilities that defy physical science. The ludicrous idea that Uhura could ever return to normal after having her life memory wiped seems to have escaped the writer's notice.
Overall, a good entry with a nice mix of action and something to think about at the same time.