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In the opening scene, Kirk and Spock are quizzing Chekov about his studies on Klingons and the region of the Federation they are in. We find out about the Organian Peace Treaty; it's not stated, but presumably this was put into effect when the Organians stopped a war breaking out between the Federation and the Klingons in season one's "Errand of Mercy". Under this treaty, planets in dispute are awarded to the side that can develop them most efficiently. The current disputed planet is Sherman's planet. This scene is a nice way to inform the audience while also giving some quiet character moments and reinforcing Chekov's novice status.
The Enterprise receives a priority one distress signal from spacestation K7 near Sherman's planet. The Enterprise rushes to the spacestation, only to find no emergency. This starts the whole situation off on the wrong foot with Kirk. A Federation official, Nilz Baris (and his assistant Arne Darvin), wanted the Enterprise to rush here to guard some experimental grain that will be used on Sherman's planet. Only on Spock's urging does Kirk assign two guards. Then he authorizes shore leave on the spacestation.
Meanwhile, Uhura and Chekov have visited the station's bar as part of shore leave. There they find the dubious businessman Cyrano Jones trying to sell goods to the bar's owner. Jones has a tribble, which is a small furry animal with no apparent limbs or sensory organs. All it does is purr when you pet it, and eat. After Uhura admires it, Jones gives it to her (for good publicity), and then sells a number to the bar's owner.
This is a fun scene, because how often to we get a look at the junior officers' private lives? We again get the feeling of Chekov's naivete, as Uhura is apparently showing him the ropes around the station.
Soon after, Kirk rushes back to the spacestation when Klingons arrive. They are commanded by Captain Koloth, who claims they are there only to invoke shore leave rights under the peace treaty. Kirk can't deny them, but requires that there are no more than 12 Klingons on the station at once and details an equal number of security guards from the Enterprise. Koloth is a strange Klingon compared to those we've met in previous episodes; he's unusually outgoing and almost affable, in an oily way. I think he would make a good used-car salesman. His second in command seems much more devious.
Back on the Enterprise, we learn that Uhura's single tribble has turned into about a dozen - apparently "she" was pregnant when Uhura got her. She gives away the offspring, including one that McCoy takes to study. Baris calls from the station, aghast that Kirk has let Klingons onto the station, giving Kirk one of my favorite replies, "I was not aware, Mr. Baris, that twelve Klingons constitutes a swarm." A flock of birds, why not a swarm of Klingons? Kirk's hands are tied on the situation, but he does take the opportunity to insult Baris.
This brings us the the most famous and best two scenes in the episode: the fight in the bar, and Kirk's investigation of it afterward. Kirk has forced Scotty to go on shore leave to the station - who can blame Scotty for resisting when the last time he was on shore leave, he was accused of a triple homicide ("Wolf in the Fold")? Scotty joins Chekov and other Starfleet crewmen in the bar.
Klingons are also drinking in the bar. Koloth's second in command decides to begin taunting the Starfleet crewmen. He begins by insulting Kirk, uttering the legendary "swaggering, tin-plated dictator with delusions of godhood" line. This gets Chekov angry enough for a fight, but Scotty holds him in check, since they are "big enough to take a few insults." There are some great details here, like when Scotty switches Chekov's empty vodka glass with his own glass of Scotch, and when Chekov takes a drink, he makes an awful face.
The Klingon realizes that his insults aren't getting the results he wants from Scotty, so he begins insulting the Enterprise instead. When he says the Enterprise "should be hauled away AS garbage", Scotty breaks and attacks the Klingon. Quickly, a brawl erupts in the bar. The Starfleet men and Klingons seem to be pretty evenly matched, but the cherry on top of this scene is watching Cyrano Jones take advantage of the bar owner's absence to make a few drinks for himself and leave the fray with one drink intact.
Back on the Enterprise Kirk has all of the crewmen involved in the fight lined up at attention. He demands to know who started the fight, but no one is talking. I read this as not only solidarity with their fellow crewmen, but also because the highest-ranking officer there was the one that started the fight. Kirk dismisses them all (confined to quarters) except Scotty, hoping to get the truth from Scotty. Eventually Scotty reveals how he resisted the fight until the Enterprise was insulted. The look on Kirk's face throughout Scotty's revelation is priceless; he was almost proud that the crew would fight with his honor, then somewhat chagrined at the fact that for Scotty only the Enterprise's honor was enough. Even the music here, a Scottish march of some kind, punctuated the meaning. And in the end, Scotty's punishment lets him get back to what he really wanted to do in the first place - read technical journals!
The tribble situation on the Enterprise has become serious, with tribbles everywhere - even somehow sticking to the walls. Kirk orders them removed from the ship. McCoy says that from what he can tell, the tribbles are born pregnant, "which seems to be a real timesaver." Eventually, they realize the tribbles have gotten into the ships ventilation ducts, which are just like the ones on the spacestation. They rush to the station.
When Kirk opens the grain's storage compartment, he is inundated with tribbles. They have eaten all the grain. Baris is beside himself with anger. In short order, though, Spock and McCoy realize that most of the tribbles are dead. Evidently the grain has been poisoned. Thus, the tribbles saved the project on Sherman's planet! Kirk calls for Cyrano Jones to be brought to him.
In the station manager's office, Kirk, Baris and other question Jones. However, it's not clear that he has committed any wrongdoing. Koloth arrives, demanding an apology for how his men have been treated on the station. Jones begins to leave, but his tribble screeches as he goes past Baris's assistant, Darvin. Since tribbles have previously only screeched when near Klingons, everyone is suspicious, and McCoy's medical instruments confirm that Darvin is a Klingon. He then proceeds to break down in the gentlest interrogation ever: being put near screeching tribbles. Hopefully for him he won't be sent back to the Klingon empire after his trial, since I'm sure they won't be so gentle!
Darvin reveals that he poisoned the grain, and his plan to ruin the Federation's effort on Sherman's planet would have worked if it weren't for you nosy kids! - I mean, the tribbles. Baris swallows his anger at Kirk upon realizing his assistant was a Klingon agent. Kirk summarily expels the Klingons from the station and Federation space. His last act on the station is to order Jones to remove every tribble from the station (which is now nearly inundated) or face twenty years in a rehabilitation colony.
In the final scene, Kirk returns to the Enterprise to discover all the tribbles are gone. When he asks what happened to them, McCoy, Scotty, and Spock pass the buck repeatedly in the explanation. Finally Scotty reveals that they beamed the tribbles to the engine room of the Klingon ship. This is great for a laugh as one imagines the Klingons suddenly inundated with tribbles. However, given that Kirk was aghast at the thought of killing the tribbles by beaming them into space, does he really think this is any better? After all, the Klingons had no problem massacring hundreds of Organians (in season one's "Errand of Mercy"), so surely they will just immediately kill the tribbles.
This episode is a lot of fun, mostly due to the tight writing. Something is always happening, even though the events at the time might not seem to be so important. The verbal exchanges (between Kirk and Baris, Kirk and Koloth, Spock and McCoy) seemed exceptionally barbed, although the humor in many cases made them more acceptable.
Spock's character development here is interesting. He is constantly at Kirk's side (paralleling Baris's aid and Koloth's second in command), providing understated support for and even restraint on Kirk's actions. When Kirk gets hot under the collar from Baris, Spock will give him a look or raise an eyebrow, which makes Kirk realize he needs to moderate his course.
As I mentioned in my review for "I, Mudd", I wish that the character of Harry Mudd had replaced that of Cyrano Jones. I think it could have been a pretty seamless swap, since the characters are very similar, but the episode would have been much more effective with the benefit of Kirk and Mudd's mutual past histories.
One incidental scene I enjoyed: when Baris is demanding that Kirk investigate Jones as a possible Klingon agent, Kirk has Spock reveal what their investigation has already found. This is very smart writing, because it shows that Kirk and Spock know their jobs; the situation on the station is important, so they obviously would look into anyone unusual like Jones. In fact, it's implied that it's so much a part of their jobs that it's not worth mentioning until someone like Baris brings it up. Nice.