Star Trek The Animated Series: Episode 3 "One Of Our Planets Is Missing"

"One Of Our Planets Is Missing" begins as a fairly standard giant space monster story, then takes a left turn about half-way through and becomes a somewhat unusual first contact  tale.
A vast cosmic cloud from outside the Milky Way galaxy has been sighted on the outer fringes of Federation space.  The Enterprise, which always seems to have the rotten luck of being the only starship in the vicinity when giant space clouds appear, is dispatched to investigate.  As the ship approaches the cloud, the crew witness it engulf and consume the outermost planet of the Pallas 14 system, then change course for Mantilles, a Federation colony with over 80 million inhabitants.
Intercepting the cloud, the Enterprise is drawn inside.  Spock deduces that the cloud is a living organism and that the ship is, essentially, inside its digestive system.  The science officer locates what he believes is the creature's brain, speculating that it may be intelligent.
With time running out, Kirk decides that the only way to save Mantilles is to destroy the cloud creature's brain.  As it turns out, however, the only way to have enough power to destroy the brain is by self-destructing the Enterprise.  Though hesitant to destroy an intelligent life form, he concludes that "the needs of the many outweigh the needs of the few, or the one," and that the lives of the millions on the planet must take precedence.  Spock convinces the captain to allow him to make a last ditch effort to attempt to communicate with the creature.  Reaching out with his mind to touch the creature's thoughts, Spock shows the creature that the planet it is about to consume contains intelligent beings like itself.  The creature, not wanting to kill, stops its advance toward Mantilles at the last possible second.   Furthermore, Spock convinces the creature that since there are many such inhabited planets in this galaxy, if it truly does not wish to consume intelligent life it should head out of the galaxy and back to where it came from.

Marc Daniels, the writer of this episode, was another Trek veteran, though this was his first time writing an episode.  His previous service to Starfleet had been as director of fourteen episodes of the original series beginning with the first one aired, "The Man Trap," and including some of the series' finest episodes, such as "The Naked Time" and "Mirror, Mirror," as well as the generally acknowledged worst episode of the series, "Spock's Brain."  Bill Norton, in his article on the animated Star Trek for the fan magazine Trek (reprinted in The Best of Trek), chided this episode for its similarity to "The Doomsday Machine," which Daniels directed and which featured a giant planet eating thing, as well as "The Immunity Syndrome," where the Enterprise encounters a giant space amoeba.  For my part, I noticed more of a similarity to Star Trek The Motion Picture, in which Spock similarly attempts to mentally communicate with an alien intelligence at the heart of a vast space cloud that the Enterprise is within and which menaces an inhabited world.  Norton, of course, could not have noticed this, as his article was written prior to STTMP's release.
I like this episode, but it has some flaws.  The dialogue, especially early on, is a little too heavy on technobabble.  Its almost enough to confuse even a hard core Trekkie like myself and was certain to have baffled the young kids watching this on Saturday mornings back in the 70's.   Also, I find the rather tongue in cheek title of the episode to be somewhat inappropriate, leading the viewer to expect a more light hearted outing than that which follows.
One thing I really like about the episode, though, is the characterization of Captain Kirk.  He is faced with a difficult moral choice, and makes it decisively, acting like a true Starfleet captain.  While he has some qualms about destroying a possibly intelligent life form, he doesn't unnecessarily agonize over it, and quite rightly decides that the lives of the millions on the planet must take precedence. 

Overall, "One Of Our Planets Is Missing" is a pretty decent and entertaining episode of Star Trek that emphasizes the Enterprises primary mission to "...seek out new life..." and "...to boldly go where no man has gone before."