"One of Our Planets is Missing" is the kind of episode (think Next Generation's "Galaxy's Child") where the ship comes across a space creature that turns out to be more complicated than the simple menace it at first appears to be (see also: "The Devil in the Dark").
During the course of the crisis, Kirk and Spock are forced to go beyond their typical Animated Series restraints and either insert their own views (Kirk's concerns about doing the right thing) or familiar behavior (Spock mind-melding with something other than a humanoid, such as in the aforementioned "Devil in the Dark" or The Motion Picture and The Voyage Home).
|via John Kenneth Muir|
"...Planets is Missing" also features the speaking debut for series-specific element crewman Arex (voiced by Jimmy Doohan), one of those elements the Animated Series could pull off but would otherwise be difficult even now to feature in an ongoing capacity (although he could probably be an interesting addition to the movies).
four quarter analysis
|via Daily Motion. Arex on the left, third arm sticking out of his chest.|
series * essential * character