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The Enterprise arrives at Sarpeidon, a planet whose star is going to go nova in 3.5 hours. I believe the episode said "nova" even though some sources say "supernova"; either way, there are major astronomical problems with this, which I'll discuss after the synopsis portion of the review. It's not clear what the Enterprise is hoping to accomplish with this visit, since they couldn't evacuate many of the inhabitants with only one ship, not to mention the time limit.
However, when Kirk, Spock, and McCoy beam down into a futuristic library, they find no one there except Mr. Atoz, the librarian, and two of his replica assistants. Mr. Atoz assures them that the planet's inhabitants are all perfectly safe, and he helped them escape. The library consists of some kind of memory disks of different parts of the planet's history that can be placed into a machine to show video of what the disk contains. After some confusion, it becomes clear that everyone on the planet was sent back in time to some previous part of the planet's history, using a machine called the Atavachron; Mr. Atoz himself is planning to go back to a specific time period to join the rest of his family.
For some reason, Mr. Atoz doesn't seem to comprehend that the Starfleet men are not native inhabitants, and that they have a spaceship so they can escape before the planet is destroyed. He keeps pestering them to choose the time period they want to escape to. McCoy randomly views a disk showing the planet's ice age, 5000 years ago. Kirk is viewing a disk that seems to be a medieval or early renaissance period when he hears a woman scream, apparently from the balcony outside the room. Never one to hesitate coming to a lady's aid, he dashes out the doorway, and disappears. Spock and McCoy hear Kirk call out and rush after him; they share a nice look of agreement that they will pursue Kirk, and go through the doorway simultaneously, also disappearing.
Kirk was sent into the time period he was viewing: a part of history similar to early renaissance Europe. He sees several men dressed in "three musketeers" garb and with rapiers pushing a woman around. He intervenes, grabs a rapier, and chases the men away. Then he realizes that he cannot see the portal he came through and starts calling out for Spock and McCoy, much to the confusion of the woman he rescued.
Spock and McCoy were sent into the ice age McCoy was viewing. It's extremely windy and cold, and they can't see the portal to the library, either. They hear Kirk yelling and respond, but the three don't get much chance to figure out what happened, because Kirk's adversaries return with the local police. The woman and some of the men hear Kirk talking to the "spirits" Spock and McCoy and believe that he is a witch; the woman herself is a petty thief. Kirk is dragged off to jail.
Spock and McCoy are rapidly being overcome by the cold conditions and begin searching for shelter. McCoy collapses and urges Spock to go on without him, which Spock absolutely refuses to do. Fortunately, a stranger in a thick fur coat arrives and shows them to a warm cave. Spock carries McCoy in and warms him up, which is about all he can do to help the doctor recover.
The stranger turns out to be a beautiful young woman named Zarabeth. She tells Spock that she was sent back to the ice age by a cruel tyrant (whose name Spock recognizes from Sarpeidon's history), because some of her kinsmen were involved in a plot to assassinate him. He didn't want anyone to think he just sent her to die, so he gave her some supplies and weapons; she has made a home for herself in the cave, which is heated by a hot spring. We don't know how long she has been there, but she is terribly lonely. She realizes that Spock is not from her planet (but space travel hadn't been developed in her time) and that it's unlikely anyone would ever visit the ice age, so she begins to believe she's going mad. Spock manages to reassure her.
Spock makes clear his intention to try to return to the library as soon as possible. Zarabeth says that they cannot go back to the future time because they will die; the Atavachron changes people to "fit in" to the time period they are going to, which kills them if they return. Spock takes this news remarkably well.
McCoy regains consciousness, and Spock eventually breaks the news to him that they cannot return to the library. McCoy can't believe that Spock is just giving up on the idea, and Spock lays into him with a verbal lashing that is outwardly emotionless, but is completely unlike Spock. We know that something is going wrong with Spock, but it's not clear at first that it's more than the stressful situation. We later see Spock having difficulty reasoning through the situation logically.
Meanwhile, Kirk's prosecutor comes to talk to him in jail. Kirk is careful about revealing information about himself, but when he mentions the library, it's clear that the prosecutor knows this special location: he is also from the future. Kirk demands help in returning to the library, but the prosecutor panics and flees amid cries of "witch" from Kirk's "accomplice".
In the ice age, Spock and McCoy have another confrontation over their fate; when McCoy calls Spock a "pointed-eared Vulcan", Spock actually picks McCoy up by the collar and says that he's never really liked being called that, then stalks off. Spock seems willing to settle into a life with Zarabeth. She is clearly intrigued with him, and he realizes that she is in fact a beautiful woman. Although he fights his emotional urges, he eventually gives in and embraces Zarabeth. Although McCoy doesn't witness this directly, he's aware of their growing attachment and still doesn't buy Zarabeth's claim that they can't return to their own time.
In jail, Kirk manages to knock his guard unconscious and grab the keys to release himself. Just then, the prosecutor returns, so Kirk takes the opportunity to nab him and get some answers. The prosecutor confirms that he is one of the people sent back from the future, but he's very wary of helping Kirk, since Kirk has been accused of being a witch. This is very pragmatic, since the prosecutor does have to make a permanent life for himself in this society.
The prosecutor also says that Kirk can't return to the library because he was "prepared" by the Atavachron. However, Kirk says that the circumstances of his "trip" meant that he wasn't prepared at all - in that case, the prosecutor says Kirk will die if he doesn't return to his own time. The prosecutor quickly takes Kirk to the location of the portal, and Kirk feels along with walls until he finds the place where he can slip through.
Once back in the library, Kirk encounters a very irked Mr. Atoz and replicas. Mr. Atoz is so worried about getting Kirk to safety that he stuns Kirk with a weapon and loads him onto a cart to push him through the portal. Kirk rolls off the cart just in time, overpowers Mr. Atoz, and compels him to help him find Spock and McCoy. There are only minutes remaining until the star goes nova.
In the ice age, McCoy has reasoned out that Spock's behavior is somehow reverting to that of the Vulcans at that time, 5000 years before their own time. However, he has to make Spock realize that, and so he confronts Spock about giving up on their situation and accepting Zarabeth's lie that they cannot go back. He claims that Zarabeth will say anything to keep Spock with her since she has been so lonely. Spock throws McCoy against the wall and almost strangles him before he realizes he's not acting like himself. When he gains control of himself, he questions Zarabeth more carefully. Zarabeth confirms that she knows for certain that only she cannot return.
McCoy declares that he's going to try to go back and stalks off through the snow to look for the portal. Spock and Zarabeth go along. They begin looking for the portal just at the same time that Kirk (in the future) is looking through disks with Mr. Atoz to find the one for Spock and McCoy. The separate groups call out to each other at just the right time to make the connection.
Even though Spock now knows he can return to his own time, he is hesitant about leaving Zarabeth. But since McCoy and Spock arrived simultaneously, they can only leave simultaneously, and Spock finally passes through the portal with McCoy. Mr. Atoz quickly puts in his own disk and dashes through the portal to safety. The Starfleet men beam up to the Enterprise, and we see the ship leave just ahead of the star's explosion.
This episode is another hidden gem in the lackluster second half of the third season of TOS. There are a lot of interesting ideas and character development, although there are also some problems.
The interaction between Spock and McCoy is the best part of this episode. First, we see them both "get into trouble" by running to Kirk's aid, which is classic behavior for them. Then, we see Spock characteristically refusing to leave McCoy behind when he can't make it through the ice age weather. These parts of the episode help emphasize just how much Spock's behavior and character are changing as he "reverts" to primitive Vulcan behavior. McCoy is still his normal irritable and irritating self, but Spock will no longer tolerate it. I remember the first time I watched the episode and saw Spock haul up McCoy by the collar and I was flabbergasted.
As Spock's emotional control breaks down, we also get some good glimpses into how he thinks. He verbalizes his logical "equation" about whether he should remain to help McCoy or go to try to find Kirk, making literal the kind of mental deliberation he must do on a continual basis. Here, he thinks it aloud because he can no longer make the "equation" work out.
When Zarabeth asks him if he knows what it's like to be truly alone, he confirms that he does. Is this his real feeling, or just a sign of his current problems? If it's real, is that how he feels all the time, even with Kirk and the others around, or is he thinking of his life prior to the Enterprise? In any case, we see again how he does not feel himself to be truly a part of any family or group or species. Because of this, it's gratifying to see him take some joy in being with Zarabeth, and it's painful later to see him have to give it up.
At the end of the episode, even when Spock knows he's not behaving normally and that he could return to his own time, he wants to stay with Zarabeth. Is he capable of making this decision rationally? Would he really be willing to throw away his entire life to live in an ice age under primitive conditions with her? The writing here is excellent, because we get the strong impression that yes, he would choose to stay with Zarabeth: he tells McCoy to go ahead of him back to the library, and we don't think Spock will ever follow. But the writing lets us have our cake and eat it, too - Spock has essentially chosen to remain and so we see that happen. But because McCoy cannot return without Spock, we see that even in Spock's "reverted" state, he still has his basic sense of honor and duty - Spock might chose for himself to remain behind, but he knows he cannot condemn McCoy to stay behind. And so Spock gives up his potential life of happiness to return with McCoy.
As much as I appreciated the insight into Spock's character that his "reversion" gave us, the idea that Spock would "revert" has no scientific basis. There is nothing about the universe that would cause Spock's brain or cells to change just because he is in a previous time.
Similarly, there is no need to "prepare" people to live in previous times, at least not in the ways described. The prosecutor said that the Atavachron changes a person's cell structure and brain patterns to live in the previous time. Um, why? The prosecutor's time period is some hundreds of years of years prior to the present time and the ice age is 5000 years before; while it's not impossible for some evolution to have occurred during this time period, it would be minimal. Even so, there's no reason that a "modern" human couldn't live in the previous time period.
The only "preparation" that would be wise would be protection against the diseases endemic to the earlier time periods - one wouldn't want to go back 500 years to be saved from a nova only to die from yellow fever or the measles. It would also be wise for the time traveler to have any potential diseases he might carry eliminated; again, one wouldn't want to go back 500 years just to contaminate the past with some "modern" influenza, for example.
This leads me to another issue which is not addressed at all: wouldn't all these people being sent back into the past change the planet's history and thus the present circumstances? Obviously, they couldn't change the fact that their star is going nova. But if someone did the wrong thing, perhaps the time-traveling technology that saved them would be prevented from being invented. Then no one could back in time to be saved! Gotta love those time paradoxes. Clearly, the Sarpeidons thought that saving themselves was worth the risk.
It's also very interesting that Sarpeidon developed time-travel technology, but not space travel. We don't know for certain that this is the case, but one would think that evacuating the planet by ship would be a more obvious plan than using time travel (especially given the risk in the previous paragraph). Their time travel capability has been around for quite awhile, too, because the tyrant used it to send Zarabeth into the past, and yet the tyrant existed long enough ago that he's a name in a history book to Spock. This would seem to be an odd imbalance in the technological development of the civilization.
While we are discussing time travel, I have to complain about the trap that time travel episodes often fall into: why should a completely alien planet like Sarpeidon have a early renaissance stage so similar to the Earth's? The clothing, weapons, buildings, and even fear of witchcraft were just like a few hundred years ago on Earth. This is convenient in terms of props and costumes, and even may make the audience more comfortable, but it doesn't make sense.
Finally, there is the issue of Sarpeidon's star going nova. First, to clarify, when a star like the Sun dies, the last stage of its death is a nova. The outer layers of the star are blown off by a strong wind - it is NOT an explosive event. A supernova is the last stage of the death of a star much more massive (and larger) than the Sun - this is an explosive event of incredible energy. Since it's much more likely that an inhabitable planet like Sarpeidon would orbit a star like the Sun, I will assume that's the case here.
Even if the episode did state the star would go nova (instead of supernova), it's silly to have a countdown to it. Let me elaborate. A star like the Sun is fusing hydrogen to helium in its core, which is what produces the light it emits, and why we call it a "normal" star. However, even in this "normal" state, the Sun is gradually changing. The Sun is slowly becoming brighter (more luminous) and larger in diameter; the Sun will continue to do this over the next 6 billion years or so as it continues to fuse hydrogen to helium in its core.
We know that the Sun's brightness directly affects the surface temperature of the Earth, so as the Sun gets brighter, the Earth's surface will gradually get warmer. In a little over 1 billion years from now, the Sun's brightness will have increased enough that the Earth's surface will be hot enough that all ice will melt. In about 3.5 billion years from now, the Earth's surface will be hot enough that the oceans will evaporate completely. And this is all while the Sun is still a "living", "normal" star! So if Sarpeidon's star is similar to the Sun, then Sarpeidon should have become baked and unlivable well before the star even began to die.
So when does the nova happen? Following how the Sun will die, in about 6 billion years (long after the Earth's surface is a dried-out, sterilized husk), the Sun's core will run out of hydrogen, so fusion will stop. The Sun will expand into a red giant, about 160 times larger in diameter than it is today, over a period of about 1.3 billion years; the Earth will not be "eaten" by the Sun, but its surface will get so hot it will melt.
Glossing over events a bit, the Sun will then go through a stage where it fuses helium in its core, which lasts about 100 million years. Then the Sun will swell into an even larger red giant (180 times its diameter today) over a period of about 20 million years. This is when the nova will happen. The outer layers of the Sun will gradually expand outward so far that they escape from the pull of the Sun's gravity and blow off into space: when the layers blow off, that's the nova. It's not a particularly sudden or explosive event, and as you can see, the sequence of events leading up to it are bad enough for any inhabitable planets.
So, in short, the count down to the nova of Sarpeidon's star is silly and completely unrealistic, given that Sarpeidon and its star appear perfectly normal until the moment of the nova. I don't know if the mistake in the episode was due to lack of research in the writing, or the state of astronomical knowledge when the episode was written - I do not remember how much was known about the Sun's death in the 1960s. At any rate, I don't really have that much of a problem with this mistake, since the fact that it was a nova destroying the planet was pretty much an arbitrary choice on the part of the writer - a planet-wide, unavoidable disaster was needed for the plot. However, it gives me the opportunity to discuss the issue here. If you are interested in reading in much more detail about how the Sun's death will affect the Earth, I highly recommend the book Death from the Skies! by Phil Plait, which discusses many different ways the Earth could be destroyed, including the Sun's death. It's a lot of fun!