The Enterprise is invited to visit an outpost near the edge of Federation space; Kirk, Spock, McCoy, and three Redshirts beam down at the outpost commander's behest — only to discover that the invitation is a pre-recorded ruse; the outpost has been destroyed.
Almost immediately Kirk and co. are beseiged by unseen aliens. Simultaneously, the Enterprise is attacked in orbit by an alien vessel, so the away team cannot beam up. Two of the Redshirts are killed (one is allowed to die offscreen), but Kirk finds a mortar-like weapon among the rubble and manages to repel the alien attackers. Beaming back up to the Enterprise, Kirk initiates a pursuit of the alien ship, which was clearly responsible for the outpost's destruction, and he means to avenge that attack.
However, as the two ships fly through space, they are seized by a group of Sufficiently Advanced Aliens called the Metrons, who are not pleased with the war-like goings-on. But, as Sufficiently Advanced Aliens typically do, the Metrons have the solution, and Kirk abruptly vanishes from the bridge.
Kirk finds himself out at Kirk's Rock, sans weapons but with a recording device/universal translator/short-range transciever, where he is to engage in a Forced Prize Fight To The Death with the alien captain (called a Gorn, and no, it's not that kind of Gorn), with the loser's ship also being destroyed in the bargain. Kirk has no weapons but is told that the environment has been "prepared" with everything he might need to make one. Kirk quickly begins casting about for a phaser or some sort of technological gizmo as the Gorn captain (a huge reptiloid creature) comes after him. Just for good measure, the Gorn is not only bigger than Kirk, it is much stronger and tougher as well.
The Metrons, by means of their Sufficiently Advanced-ness, somehow allow the Enterprise bridge crew to follow the action on their viewscreens (and presumably the Gorns as well). As Kirk futilely fights off the Gorn, the Gorn raises the accusation that the outpost was in Gorn territory and that they thought it might be the foothold of a Federation invasion — a possibility which shocks McCoy, thinking that the Federation might actually be in the wrong this time. Despite Kirk's denial of the accusations, the Gorn keeps coming, relentless and seemingly unstoppable.
Finally, an exhausted and nearly defeated Kirk begins to notice the various mineral deposits all around, and eventually recalls that these can be used to formulate a crude form of gunpowder. Kirk quickly assembles a makeshift mortar using a bamboo shoot and crystal shards for bullets as the Gorn approaches, firing it off Just in Time. The Gorn is pierced, and falls immobilized and helpless.
Kirk is the victor and is ready to deal the death stroke... and abruptly announces to the Metrons that he refuses to kill the Gorn, whom Kirk believes was only defending his territory. An image of a Metron appears and expresses surprise at Kirk's mercy, noting that were the tables reversed, the Gorn would not be so merciful. "You are still half-savage. But there is hope."
Recapper's Note: According to Word of God, the Forced Prize Fight was indeed a Secret Test of Character, but not quite as it comes across: The script includes a bit of dialogue (not aired) which reveals the Metrons had planned to destroy the winner of the fight rather than the loser, considering the winner to be a bigger threat to them. Luckily for us, Humans really Are Special. (The line is, however, included in the novelization by James Blish.)
The opening credits say "Story by Frederic Brown, Teleplay by Gene R. Coon." Frederic Brown's "Arena" was first published in the June 1944 issue of Astounding. Gene L. Coon wrote the script without consciously remembering Brown's story. The Kellam de Forest Research Agency, which handled all ST scripts, noted the similarity. Desilu got permission from Brown, not telling him Coon had already written the script, but paying him a fair price and giving him screen credit.
- Adaptational Modesty: The original tale by Brown had the human protagonist running around in the nude. Kirk only suffers some Clothing Damage from his peril.
- Animal Eyes: The Gorn has insect-like compound eyes on a reptilian body, further adding to its abhorrent alien appearance.
- Attack Its Weak Point: In their first encounter, when Kirk is grappling with the Gorn and unable to do it any harm thanks to its strength and rock-hard skin, Kirk buys himself time and escapes by slamming his hands onto its ear holes, which clearly causes the Gorn a great deal of pain.
- Bamboo Technology: Kirk's cannon.
- Big "WHY?!": Just because you're seriously injured doesn't mean you can't pour it on.Cestus III survivor: Why did they do it? WHY?! There's got to be a reason!
- Blood Sport: Kirk accuses the Metrons of this, declaring "You'll have to get your entertainment somewhere else" after refusing to kill the defeated Gorn.
- Boulder Bludgeon: Captain Kirk and the Gorn captain are forced to fight each other with improvised weapons. During their battle, the Gorn captain picks up a boulder and throws it at Kirk, pinning Kirk's leg to the ground.
- Captain's Log: The Metrons are kind enough to provide Kirk with a recorder for this purpose.
- Catch Phrase: The Metrons insist on saying, "We are the Metrons" every time they start a conversation.
- Clothing Damage: Kirk tears his pants (in a totally non-Fanservice way) to make a fuse for his cannon.
- Crazy Enough to Work: Kirk's makeshift cannon. Future Star Trek media would end up repeatedly mentioning that Kirk got really lucky that the cannon didn't just blow up and kill him.
- Mythbusters in an episode dedicated to Star Trek tropes demonstrated exactly the uphill climb Kirk was up against. Making gunpowder is not anywhere as easy as the episode makes it look, the Mythbusters ended up having to spend a LOT more time in trial-and-error and meticulous weighing and measuring than the Gorn would have allowed to even get in the ballpark of useable gunpowder and not just a mess that wouldn't even ignite. And even if that hurdle was surmounted, bamboo, even reinforced by rope, is NOT strong enough to withstand the pressure of exploding gunpowdernote . Mythbusters showed a near 100% chance that the weapon would have been more deadly to Kirk than to the Gorn. When the Mythbusters set the weapon off (using their trusty guinea pig, Buster), it promptly exploded in a manner that would have shredded an actual human being beyond hope of survival. It was only when they finally cheated in the last try and reinforced the weapon with a metal interior that they had a confirmed Gorn kill.
- However, in the episode the bamboo, mineral deposits, and diamonds were all placed by the Metrons specifically so they could be used to construct weapons. It's quite possible the bamboo was stronger than earth-variety bamboo, and the mineral deposits more pure and more easily combined to create gunpowder.
- Dangerous Forbidden Technique: Kirk's orders to increase speed to Warp 7 and then Warp 8 are played up as risk-taking that seriously worries the bridge crew.
- Depleted Phlebotinum Shells: Kirk drops a small blue ball into a mortar-like device, the resulting blast being enough to drive off their attackers.
- Disproportionate Retribution: We're meant to think the Gorns may have been justified in repelling what they saw as an invasion of their territory, but this kind of ignores their killing helpless women and children. It's made worse when you consider the fact that the Gorn can understand our language note , meaning that they decided to kill everyone at the outpost without trying to communicate with them and knowingly refused their attempt to surrender.
- Dramatically Missing the Point: The Metrons state the planet has materials with which the combatant's can fashion weapons. Kirk mishears or misremembers this as the Metrons saying there were weapons, and spends the bulk of the episode running around, hoping to find a phaser under a pile of sand or a sword stuck in a rock. Meanwhile, the Gorn captain, having paid more attention, has fashioned a crude stone knife, vine booby-traps, and a net.
- Early Installment Weirdness:
- A minor example—the sound effect of the Enterprise firing phasers is different from what it will end up being.
- An aversion, as this is the first mention of a "Federation" that Kirk works for. It would be fully named in "A Taste of Armageddon."
- Explosive Instrumentation: A literal version when the Gorn remotely overload Spock's tricorder, and he has to throw it away before it blows up in his face.
- Forged Message: The Enterprise is sent a faked transmission to lure them into beaming down with their Tactical personnel. Everyone ignores Spock questioning this Schmuck Bait.
- Forced Prize Fight: The plot.
- Forced to Watch: The bridge crew are shown a transmission of what Kirk is undergoing, though this is presented as a Pet the Dog rather than the usual Kick the Dog.
- Gadgeteer Genius: Kirk shows hints of this when he cobbles together a crude cannon out of the elements in the landscape.
- George Lucas Altered Version: The remastered version includes a few shots of the Gorn blinking and images of the Gorn ship on the main viewscreen.
- Humans Are Special: The Metrons' justification for sparing the Enterprise.
- Insult Backfire: Spock accuses Bones of being a sensualist when the doctor mentions how much he's going to enjoy having non-reconstituted food. "You bet your pointed ears I am!" the good doctor says proudly. It isn't often one of Spock's insults backfires on him!
- It Can Think: The Gorn captain initially doesn't speak and relies on his physical superiority in the first round, leading Kirk to believe he's little more than a brute. However, he was actually paying more attention than Kirk was when the Metrons explained the rules and was just keeping his mouth shut while Kirk was blabbing all of his plans on the recorder/communicator, allowing him to turn Kirk's ambush attempt into bait for his own trap. This includes a rather amusing scene where he's listening to Kirk blabber about using human brains to beat Gorn brawn, and the actor gets a rather articulate insulted/amused look out of the inert mask.
- In the original script, which was used and expanded on by James Blish when he wrote the adaptation, the Gorn Captain and Kirk have several extended conversations (cut out from the episode for time and pacing). During one conversation, the Gorn Captain says that they have to finish this quickly, for their own sake as well as their crews'. There's no water to be seen, and the Gorn Captain also notes that he doesn't see anything he can eat... with the possible exception of Kirk. Damn.
- The Juggernaut: The Gorn.
- Late to the Tragedy: The outpost was destroyed at least a day before the Enterprise's arrival.
- Lizard Folk/The Reptilians: The Gorn again.
- Locking MacGyver in the Store Cupboard: The Metrons announce that the planet has been stocked with "everything [the fighters] might need to make [a weapon]." Luckily, our good captain is extremely resourceful.
- The Main Characters Do Everything: A striking example: the Enterprise is under attack in orbit while Kirk is planet-side. Although Kirk is himself under fire, he takes time out to micromanage the space battle over the communicator (with such insightful tactics as 'fire phasers' and 'fire torpedoes'), rather than just letting Sulu do his job.
- Mighty Glacier: The Gorn may be much stronger than Kirk, but he is also much slower.
- Moral Myopia: The Gorns and the Enterprise crew intend to inflict violence on each other, and the Metrons won't have it. So they send Kirk and the Gorn captain to the planet's surface so they can inflict violence on each other until one is dead, so the Metrons can make his entire crew dead. Because killing is bad.
- No Name Given: The Gorn Captain's name is never mentioned. Some Expanded Universe sources gave him different names: either S'alath, S'slee, S'salk, Rheuzz'r or Arijog.
- No One Gets Left Behind: Averted; Kirk orders Sulu to do a Hyperspeed Escape, abandoning himself and the other crewmen who are under attack on the surface. Fortunately the Gorn retreat before Sulu carries out the order.
- Obscured Special Effects: We never see the Gorn attackers on the planetary surface, as they're firing shells and disrupter weapons from behind a ridge. This actually adds an air of realism to the scene, similar to how a contemporary military force would use snipers and indirect mortar fire to soften up an enemy before launching an attack.
- Only Smart People May Pass: The Metrons make it clear that the planetoid has all elements necessary to manufacture weapons... including the minerals that are elemental to create basic gunpowder, if someone knows chemistry. The original story by Brown makes it much more specific, by means of the same planetoid having various obstacles like a forcefield that kills anything living that tries to cross it if conscious (meaning the protagonist must figure out a way to go through it while unconscious).
- Otherworldly and Sexually Ambiguous: The Metron who talks to Kirk at the end has an androgynous appearancenote and wears a shimmering dress-like garment.
- Punch! Punch! Punch! Uh Oh...: Kirk lands some pretty good blows on the Gorn, but he's less effective than usual.
- Really 700 Years Old: The Metron that Kirk deals with is 1500 years old.
- Redshirt: The first man to die is wearing a red shirt. A yellow-shirted man dies offscreen.O'Herlihy: Captain, I see something! (vaporizes)
- Reptiles Are Abhorrent: Kirk admits "Like most humans, I seem to have an instinctive revulsion to reptiles."
- Revenge: Kirk's goal for the first part of the episode. When given the opportunity to take it, however, he refuses.
- Scifi Writers Have No Sense Of Scale: At the end of the episode, Sulu states that they've been thrown 500 parsecs to the "other side of the galaxy." While 500 parsecs is certainly a considerable distance, our galaxy spans thousands of parsecs. The Federation itself in future Star Trek canon is said to span over 8,000 light years, or several times 500 parsecs.
- Sssssnake Talk: The Gorn says "I weary of the chasssse. Wait for me. I ssshall be mercccciful and quick."
- Sheathe Your Sword: Kirk refuses to kill the Gorn. The Metrons commend him for his decision, saying that there might be hope for mankind yet.
- Spider-Sense: How the hell does Spock know that it's potassium nitrate on the rocks, and not sugar, or cocaine? Is Vulcan vision equipped with a mass spectrometer?
- It could simply be Spock making an educated guess based on logic. Kirk is collecting the mineral, and the Metrons have said that there are materials on the planet's surface which can be used to fashion weapons, therefore the mineral must serve a role in this. A weapon uses an energy source, something to contain the reaction and possibly a projectile. The only one of those that the powder could be used for — unless the Metrons expect Kirk to find the time and resources for primitive industrial processes — is an energy source. That narrows it down to a weapon which uses chemical reactions for energy and can be used against the Gorn, and the powder to one of the reactants used. It's not too long of a shot to say that the weapon is a rudimentary cannon and from there Spock can simply narrow the powder down to whichever reactant fits the appearance.
- Sword over Head: Kirk, in this position with the Gorn, refuses to kill him.
- Viewers Are Morons: Wait... what's the hardest substance known to man?
- Worst Aid: The survivor Kirk and the others find is left with his head propped up against the wall. If he fell unconscious (quite likely as he has serious injuries) he'd die of asphyxiation.
- Worthless Yellow Rocks: Kirk runs into a huge bunch of diamonds while running away from the Gorn, and deems them useless... at least at first.Kirk, narrating into the recorder given to him by the Metrons: A large deposit of diamonds on the surface. Perhaps the hardest substance known in the universe. Beautifully crystalized and pointed, but too small to use them as a weapon. An incredible fortune in stones... (tosses them away) and I would trade them all for a hand phaser, or a good solid club.
- Wowing Cthulhu: During the first encounter with the Gorn, the Enterprise and the Gorn ship are stopped in space by a mysterious and powerful race called the Metrons, who take Kirk and the Gorn captain to a deserted planet to fight to the death. When Kirk finally gains the advantage, he refuses to strike the final blow, which surprises the Metrons, who finally manifest before him and admit they had not expected him to have the advanced trait of mercy.
- You Called Me "X"; It Must Be Serious:Spock: Destruction of the alien vessel will not help that colony, Jim.