The Changeling
A mysterious probe called Nomad boards the ship, resembling one sent out from old Earth but far more powerful. It's new directive puts all organic life in peril, starting with the crew members.
This story receives a lot of attention for having inspired The Motion Picture. But while the film uses the plot elements as a basis for wondrous effects and subtle reflections on man and machine, this episode is average fare. The most interesting moment comes when Nomad is beamed aboard, and the officers believe it to be a ship containing small beings. But nah. From this point on it's clear redshirts will die, the ship will threaten to explode, and Kirk will talk the thing to death. (Though it's one of his more logical arguments, receiving even Spock's compliments.) The few surprises are Scotty's death and "repair," Uhura's memory wipe (mainly nonsensical) and Spock's mind meld, again perfectly executed by Nimoy. The Kirk/ Roykirk thing is a stretch, but a device needed to keep Nomad from instantly destroying the ship. Alas, they destroy Nomad, in one of the less positive traditions of Star Trek. So many strange, new things encountered and found to be a danger, and eliminated. Sigh.
Spock Saves the Day: Through research and mental probing, Spock uncovers many useful facts about Nomad.
Oops: Nada. Nomad finds Spock "well-ordered," so we must agree.
Developments: Spock's mind melds are not limited to things biological, apparently. He makes a similar effort with V'Ger in The Motion Picture. The Vulcan also understands Kirk's logic. It's often thought the two are at opposite ends of the emotional spectrum, but actually, they share traits like the ability to reason and willingness to take calculated risks.