#18: The Ultimate Computer
In case you forgot: An experimental AI takes over the Enterprise. Kirk saves the day by getting it to self-destruct.
This is one of the few "man versus machine" TOS episodes I really, really like. Maybe because it came right after the turd bucket that was "Omega Glory" and that made it seem that much better, I don't know.
The threat that this machine, the M-5, entails is also better-done than in other episodes. Instead of the machine working perfectly, the threat is that the machine is pretty much straight up malfunctioning, to a degree previous computers on TOS would never do. They may be obsessive control freaks that micromanage peoples' lives even to the point of the people's detriment, but it's pretty safe to say they would never blow up a supply ship for no reason at all.
More than just "man versus machine," this episode is also great because we get a great sketch in our guest star of the week, Dr. Daystrom. First of all, some color-blind casting going on in the part, which is nice. Second of all, it's a great "what if" look at the bright young genius, all grown up. What happens to a person who gets too much praise too early? Daystrom has enough background that you could write a novel or movie about him without too much more than the episode lets on: not every walk-on character of the week gets that much. (Heck, even some regulars don't even get that much.)
While it's part of Daystrom's tragic flaw that he's intent on protecting the M-5 from destruction or disrepute, for someone who's equally concerned with pacifism and nonviolence, he lets his computer baby go a bit too long in its killing spree for you to really entirely like the guy.
Spock's assertion that a starship runs on the crew's loyalty to one man really rubs me the wrong way. It's just a little too close to cult of personality for my liking.