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The Enterprise is visiting Elba II, an uninhabited planet except for a small colony housing an asylum for the incurably, criminally insane. The asylum has about a dozen patients, which Spock remarks later is an amazingly small number given the number of beings in the Federation. Their mission is to deliver a new medicine that may cure these last few patients. The asylum is well-protected: the planet's atmosphere is poisonous, and the entire planet is surrounded by a forcefield.
When Kirk and Spock beam down, they are met by Governor Cory, who greets them warmly and has apparently met Kirk in the past. Cory announces that one of their patients is Garth of Izar, a former fleet captain in Starfleet who went insane. Garth's military "exploits" are legendary, so Kirk asks to see him. Cory happily shows them to Garth's cell, where Cory is imprisoned.
We learn that the man we thought was Cory was actually Garth - he has the ability to change his appearance. Cory says that Garth was taught the ability to change his appearance by the beings on Antos; Garth had been critically injured and the beings taught him the ability so he could help heal himself. This also seems to be when he went insane. No one at the asylum knew about the ability until Garth used it recently to trick a guard into letting him out of his cell. Inexplicably, the ability seems to also enable Garth to change his clothing's appearance.
Garth releases the other inmates, who follow him devotedly. He plans to take control of the Enterprise and use it to conquer the galaxy; he fancies himself to be "Lord Garth" and "Master of the Universe". He quickly puts on Kirk's appearance and contacts the Enterprise . He cannot get himself beamed aboard, because he cannot answer Scotty's chess problem; Kirk had set up the code question and answer before he left the Enterprise as a precaution. This communication lets the rest of the Enterprise crew know that something is up on the planet, but with the forcefield in place, they cannot do anything about it, despite spending the rest of the episode working on it.
Furious at his failure, Garth tries a different approach. He invites Kirk and Spock to dinner, apparently trying to get on their good side. He and his "consort" Marta (a fellow inmate who is a green Orion woman) get into irrational arguments during the dinner. (I did enjoy the exchange: "I'm the most beautiful woman on this entire planet!" "You're the only woman on this planet, you cow!") Marta dances for them and can't seem to keep her hands off Kirk.
Spock's ruthless logic in questioning Garth's plans annoys him, so he has Spock taken away. Then he brings out a therapy chair which he has altered to emit ultrasonic waves as a form of torture. He asks Kirk for the answer to the chess problem; when Kirk refuses he orders Cory put into the chair and turns it on for a bit. Then he orders Kirk put into the chair. Marta is bothered by the torture of Kirk and persuades Garth that she can get the information out of Kirk.
Kirk wakes up in a bedroom with Marta. She claims that she's in love with Kirk, and Kirk tries to exploit this angle to get her help contacting the Enterprise for help. Marta says she's arranged for Spock to be brought to them. But you just can't trust a crazy Orion woman, and she tries to stab him. Kirk fends her off just as Spock shows up.
Spock has phasers for both of them, and they head to the control room to lower the forcefield and beam down a security team. Spock's mannerisms seem normal, but he seems overly concerned about Kirk's safety. He insists that Kirk beam up to the ship right away, while he waits to direct the security team. Fortunately Kirk is not a complete idiot, and when the time comes for him to answer Scotty's chess problem, he defers to Spock. Of course, Spock cannot reply, because he's really Garth. Garth turns the forcefield back on and cuts off communication with the ship.
While Garth has not given up, he decides to take the time to have his "coronation", and he names Kirk his heir apparent. The other inmates set up a pitiful display of a throne room, and Garth crowns himself and makes Marta his consort, giving her a necklace. Then it is time for the entertainment: Garth takes Kirk back to the control room, where there is a window to the outside. He then has Marta thrown outside into the poisonous atmosphere. He says that he has invented the most powerful explosive ever, and that there is a grain of it in Marta's new necklace. He detonates the necklace remotely, and Marta disappears in a reasonably impressive boom.
Obviously, Garth is extremely dangerous with his possession of this explosive. However, he doesn't try to press this point to his advantage. For some reason he decides that he might be able to strike a deal with Spock, and orders that Spock be brought to him. When Spock is released from his cell, he nerve-pinches the guards and takes their phaser. Garth sees this on the monitoring system, so by the time Spock arrives at the control room, there are two Kirks.
Spock cannot tell which Kirk is the real Kirk. This makes for the most frustratingly illogical part of the episode, especially since it's involving the logical Vulcan! Spock asks a question about something he and Kirk did in the past, but the answer is way too easily faked, as one of the Kirk points out. Does he ask something more personal or classified? No. He decides that Garth must be using a lot of energy in the deception, so he'll just wait. This is so silly! Stun both Kirks - chances are that when Garth is unconscious, he'll revert to his true form. Even if he doesn't, they'll both be under control and surely tests could be done on them to decide which one is the real Kirk.
Instead, the Garth-Kirk gets the drop on Spock and knocks him over; the two Kirks get into a fight quickly enough that Spock can't tell which one is which. Does Spock stun them now? No. At the climax of the fight, the Garth-Kirk shrieks a proclamation that he's the real Kirk, so Spock should stun the other one. The real Kirk agrees with the stunning, saying it will keep the Enterprise safe. With that, Spock can tell who is who and stuns Garth-Kirk, who does in fact go back to his natural form.
The next thing we know, Enterprise crewmen are in the facility and McCoy is helping administer the new medicine to the patients. Garth is starting to recover and doesn't remember anything about the recent events with Kirk and Spock.
This episode is basically pointless, and it's annoying, too. The half-star rating I gave it is purely because of Kirk's preparation with the chess problem code, which was logical under the circumstances.
The characters of Garth and Marta did a good job of acting insane. Unfortunately, it was also extremely grating to listen to them most of the time, with their fighting and irrational comments. We have no idea what Marta's background and crimes were. We know that once Garth went insane, he ordered his crew to commit genocide, which they refused to do. Why did Garth's insanity put him on a course toward galactic domination? We have no idea.
Garth apparently learned his shapeshifting ability. From comments in the episode, we know he is humanoid but not human. Even so, could humans learn this ability? It seems like Starfleet would be extremely interested in it, but this is not mentioned. I have already noted how silly it is for his ability to also affect his clothing. In addition, he seems to gain way too much information about the people he is imitating. How did he know about Cory and Kirk's previous meeting(s)? One might think he tortured Cory for this information, but then his imitation of Spock's mannerisms and speech patterns was excellent and there wasn't time for the torture of Spock.
There were a number of opportunities here that were not taken advantage of. Kirk says the Garth was a hero of his - why? More specifics about Kirk's or Garth's past would have been nice. As I mentioned above, when Spock must choose the real Kirk, it was a ripe opportunity for some revelation of an anecdote or incident in their past that could have been interesting.