Wow, where to start on this episode? It opens with them in the wrong place. Instead of being on the planet talking about how they only have 30 minutes before they have to leave to go deflect the giant asteroid which will destroy all life there, they should be out there deflecting the asteroid. And then after noting they only have 30 minutes, they look around some more.

Then when Kirk goes missing and Spock says they need to leave immediately, McCoy protests about leaving Kirk behind, and Spock spends several minutes doing an As You Know speech to McCoy about basic planetary mechanics, explaining in great detail why they need to leave immediately - before they leave.

After failing to deflect the asteroid, Spock spends his time decoding the symbols on the obelisk. He concludes that they are not words, but musical notes! Then he says, "The tones correspond roughly to an alphabet."

The message on the obelisk refers to a precursor race known as the Preservers, who seeded primitive cultures throughout the Galaxy, so they could grow and flourish without the danger of being confined to one world. This goes a long way to explaining why so many uncontacted alien worlds have humans on them - but the Preservers are never mentioned again in any episode of any incarnation of Star Trek. This would have been an awesome plot arc element to have brought up now and again, but no, it's promptly forgotten.

The Preservers were kind enough, however, to leave behind the asteroid-deflecting obelisk to keep the humans on this planet (named "Amerind", I kid you not) safe from asteroid impacts. Only they didn't bother making the obelisk work by itself. They had to train the humans how to use this piece of technology they didn't understand to deflect asteroids, and then tell them to pass the knowledge down the line of medicine chiefs from father to son. The result, inevitably, is that Goro (the elderly medicine chief in the episode) never learnt the secret because his father died before he could tell him!

The obelisk, by the way, opens to a sonic command signal (as hinted by the musical notes that are "not words"). That command signal just happens to be triggered by Kirk opening his communicator and saying, "Kirk to Enterprise." Which is how he ends up falling inside and stumbling into the amnesia beam.

Wait. Why did the Preservers, who wanted the medicine chief to remember how to use the obelisk, put an amnesia-inducing beam inside it??

Never mind. Why did they make the sonic key signal someone saying the words, "Kirk to Enterprise"??

Oh, and I haven't even started on the implications of Kirk being the naturally superior Great White Hero who saves the primitive "American Indians".

Yeah, not a huge fan of this episode.

Transcript

1
Captain's log, Stardate 4842.6. Wasting time by scouting a planet before we deflect a killer asteroid from hitting it.
Kirk: We've got 30 minutes. Let's look around some more!
Caption: Mysterious alien obelisk
Caption: Pine trees
2
McCoy: American Indians?
Spock: Yes. A mix of Navajo, Mohican, and Delaware.
3
Kirk: What are the odds of such an Earth-like world?
Spock: Quite high. Every second world we visit, really.
4
Kirk: Let's go deflect that asteroid. But first I want to do something stupidly risky and explore that obelisk alone.
5
{inside the obelisk}
Caption: Kirk is trapped inside!
Caption: Leans on buttons!
Caption: Amnesia beam!
6
{outside}
McCoy: We gotta find Jim!
Spock: I must take 5 minutes to stress the imperative to deflect the asteroid as soon as possible. Then we go, without leaving a search party to look for the captain.
7
{Enterprise bridge, deep space}
Sulu: We were too late to deflect the asteroid!
Spock: I shall study the alien obelisk as we limp back to the planet. At exactly the same speed as the asteroid.
8
{on the planet}
Caption: Meanwhile...
Miramanee: You come from the temple, and have white skin. You must be a god!
9
Miramanee: I shall marry you and bear your child!
Caption: Jealous ex-fiancé medicine man, supplanted by Kirk
10
Goro: Kirok! You are a great white god! Save us from the falling sky!
11
{by the obelisk, over-acting}
Kirk: I am Kirok!!
12
Spock: {mind melding} You are James Kirk. This obelisk is an asteroid deflector.
McCoy: And your pregnant native wife is dead, neatly tying up all the loose ends.