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The Enterprise is on a mission to stop an asteroid from colliding with a planet that has an exceptional similarity to the Earth. Kirk, Spock, and McCoy visit the planet and the humans delight in the terrestrial plants and topography. Spock remarks that with the planet's bulk properties, it's extremely unusual for it to have developed so similarly to Earth. The native peoples even seem to be a mix of Native American cultures, and have not progressed in technology beyond that of the Native Americans when the Europeans began colonizing the Americas.
They come upon a large, metallic obelisk with strange symbols on it. Spock declares that it's far too advanced for the natives to have constructed. When Spock and McCoy are out of sight, Kirk activates his communicator and accidentally opens a hatch into the room under the obelisk. He falls in and then hits a button that causes the mechanism in the room to zap him and knock him unconscious.
Spock and McCoy cannot find Kirk. Spock does not allow them to search for long, since the Enterprise must leave to get to the proper location to deflect the asteroid. McCoy is outraged, but without Kirk, Spock is in command.
The Enterprise strains its engines to get to the deflection point in time. The plan is to use the ship's deflectors to push the asteroid off-course. Unfortunately, the deflection is not sufficient. Spock orders plan B, which is to use the ship's phasers to break up the asteroid at its weakest point. This plan also fails, and the ship's engines are crippled. The Enterprise can still maneuver on impulse power, so they head back toward the planet; after about two months, they will arrive just hours before the asteroid will hit. McCoy bitterly blames Spock for failure on a number of issues: leaving Kirk behind, stopping the asteroid, and now not begin able to get back to the planet quickly to find Kirk.
I have to pause here to discuss the asteroid-impact plot. This is a remarkable plot for several reasons. First, at the time this episode was written, it had only been a decade or so since the idea that many of the craters on the Earth were the results of impacts by pieces of asteroids or comets. Second, even given that fact, very few scientists even thought that impacts could be a threat in the present day. Finally, since no one had really conceived of the threat, no one had really thought about what to do to stop an imminent impact.
I also have to applaud how parts of this plot were developed. As contemporary astronomers know, the best way to prevent an impact is deflect the incoming object off-course. The most crucial aspect of this approach is that the deflection is easier the sooner it is done. As Spock explains, if the asteroid is deflected far away, the angle of deflection needed is a lot smaller than if the asteroid is deflected close to the planet. This approach also does not require physically destroying the asteroid, which is good, because that would take an incredible amount of energy.
However, there are also some problems with how this plot was presented. Spock's plan B of breaking up the asteroid is a bad idea. Cutting up the asteroid into smaller pieces will not make those pieces change course: all of the pieces will stay in their orbit and eventually hit the planet. Whether it's better for one large asteroid or many small pieces to hit a planet is still hotly debated by astronomers today - the bottom line is that it's not a distinctly better solution.
Another oddity is that the planet in the episode is apparently expected to be frequently impacted. McCoy comments early on about how there are too few impact craters on the planet. The Preservers built the obelisk to deflect impactors, and it's apparently been used often enough that the natives are familiar with the warning signs, at least in legend. This is very odd for an Earth-type planet. One of the most recent large impact craters on the Earth, Meteor Crater in Arizona, is about 50,000 years old. The Earth just doesn't get hit that often these days. I suppose we have no idea what the planetary system this planet is in is like; perhaps it is relatively young and still has a lot of rocky debris in it.
The final problem with this plot is the size of the asteroid: Spock says it's about the size of the Earth's Moon. That's huge! In our solar system, if you took all of the asteroids and lumped them together, they would be smaller than the Moon. It's extremely unlikely to have such a large object "hanging around" in the system for so long when it's not safely in an asteroid belt. However, I write this off to dramatic license.
Back to the episode. Kirk wakes up in the basement of the obelisk and cannot remember his name or anything about his past. He stumbles out of the obelisk, and Miramanee, high priestess of the local tribe, sees him and calls him a god. She takes him back to her tribe.
The tribe elder questions Kirk, but with his amnesia, he cannot explain anything about his arrival or past. He saves the life of a nearly-drowned child using artificial respiration, and the tribe accepts this as a miracle from a god. Kirk is made leader of the tribe and medicine chief. The previous medicine chief, Salish, resents Kirk's rise to power, especially when he learns it means that he will not be marrying Miramanee as planned. Miramanee is required by custom to marry Kirk, now known as Kirok.
Within the tribe, Kirk begins to find a peace and happiness that he somehow knows he's never had. He is happy to marry Miramanee, and even Salish's attempt to kill him on the way to his wedding does not dampen his overall bliss. The two months pass quickly. We learn that Kirk has introduced a number of technological improvements to the tribe, such as oil lamps and irrigation. Miramanee announces that she is pregnant. His only discomfort is that he has dreams of a "lodge in the sky" and people on it that he thinks he should know.
On the Enterprise, we learn that McCoy has finally forgiven Spock for leaving Kirk on the planet. Now he is concerned because Spock seems to be obsessed with the images of the obelisk that they had made during their visit. Spock is hardly eating or sleeping; he is convinced that the symbols on the obelisk are a language, and that if he can decipher it, it will lead him to technology on the planet that can deflect the asteroid.
Eventually he is successful and determines that the obelisk is an asteroid deflecting device put on the planet by beings calling themselves the Preservers. The Preservers have visited many planets around the galaxy, taking "samples" of primitive cultures that seem in danger of dying out and then placing them on other planets. I guess they also alter the new planets to suit the seeded cultures, which would explain the planet in the episode. McCoy remarks that he always wondered why there were so many humanoid races in the galaxy. I'm not sure whether I think this is clever or convenient on the writer's part. At any rate, Spock now hopes to be able to use the obelisk to deflect the asteroid once they reach the planet.
On the planet, the Preservers' asteroid impact warning system goes off: strong winds and storms begin suddenly, and the skies get dark. We had previously gotten hints and comments about this method the Preservers used to notify the medicine man that it was time to use the obelisk. Now Miramanee and the rest of the tribe expects Kirk to go to the obelisk and use it to save them. The problem is that Kirk has no idea what to do to use the obelisk.
He goes to the obelisk and flails around helplessly. Salish observes him and rouses the rest of the tribe into a mob to attack him for his failure to save them. They begin stoning him, and when Miramanee joins him, they stone her, too. Just as they are both beaten down by the stones, Spock and McCoy beam down and scare the natives away.
Kirk is injured and still doesn't recognize them, so Spock is forced to mind-meld with him. Interestingly, Kirk's mind is so strong that Spock finds it hard to break the meld. He is successful in restoring Kirk's memory and even conveys the current situation to him. Kirk quickly pieces together what he was doing when he originally activated the obelisk and does it again. He and Spock enter the obelisk, and Spock quickly reads the symbols to determine how to deflect the asteroid. He activates the obelisk, and the asteroid is successfully deflected. I can't even begin to express how large an amount of energy would be necessary to do this when the asteroid is so close to the planet.
The planet and the people on it are saved, but Miramanee suffered fatal internal injuries during the stoning. In possibly the saddest scene of TOS, Kirk sits by her side as she dies peacefully in their hut.
This is a very bittersweet episode, which makes it very effective. We see Kirk find complete happiness and contentment. The scenes between him and Miramanee do an excellent job of conveying their joy and satisfaction in their relationship and their lives. We can really feel how carefree and relaxed Kirk is. Of course, we know the price of his happiness: the loss of his prior life, and friends, and command. If he could somehow choose one life or the other, which would he choose?
Unfortunately (and perhaps conveniently), circumstances make that choice for him. Miramanee's death robs him of the possibility of continuing his life on the planet. For once, Miramanee as a one-episode character was sufficiently developed that we really felt for her as she died. Despite this, even if Kirk wanted to stay on the planet, the mob stoning him should have ruined his ties with the tribe. The recovery of his memory means that he knows what his life was like before and what he had with Miramanee; reconciling the two must be extremely difficult.
Spock gets his own does of misery in this episode as he does everything he should do to complete the mission and fails. I couldn't help thinking that if Kirk had taken all the same courses of action that somehow is famous luck and intuition would have somehow ended up in success. Illogical? Certainly. At any rate, even though he would deny it, Spock clearly feels guilt at his failures. When he obsesses over deciphering the obelisk, is he doing it to save the planet, or to save Kirk's life, since he's on the planet?
One minor detraction to the show was the depiction of the natives' lives as being relatively leisurely and pleasant. In reality, the lack of modern agriculture and technology would have made farming and hunting more difficult and time-consuming than shown. In times of good weather, the food supply might be sufficient, but there would be many times of shortages. Everyone would need to work for the survival of the tribe, instead of being able to cavort through the woods. The lack of modern medicine would mean that even minor injuries or sicknesses could be deadly, and life expectancy would be pretty low. However, as I said, I can overlook this because of the other merits of the episode.
Overall, this episode was a very different and revealing look at the two main characters.