[[caption-width-right:350:You might have heard of this one.]]

The Enterprise is invited to visit an outpost near the edge of Federation space; Kirk, Spock, [=McCoy=] and three {{Redshirt}}s 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 [[{{Revenge}} avenge]] that attack.

However, as the two ships fly through space, they are seized by a group of SufficientlyAdvancedAliens 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 KirksRock, sans weapons but with a recording device/universal translator/short-range transciever, where he is to engage in a ForcedPrizeFight [[DuelToTheDeath To The Death]] with the alien captain (called a Gorn, and [[JustForFun/IThoughtItMeant 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 [[SuperStrength much stronger]] and [[NighInvulnerable 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, [[TheDeterminator 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 JustInTime. 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. [[ProjectedMan 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 [[HumansAreBastards half-savage]]. But there is hope."

[[AC:Recapper's Note:]] According to WordOfGod, the ForcedPrizeFight was indeed a SecretTestOfCharacter, but not quite as it comes across: The script includes a bit of dialogue (not aired) which reveals [[DoomAsTestPrize the Metrons had planned to destroy the]] '''[[DoomAsTestPrize winner]]''' [[DoomAsTestPrize 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.)

Compare to "[[Recap/BlakesSevenS1E8Duel Duel]]", an early episode of ''Series/BlakesSeven'' that is practically a remake of this story.

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 [[https://en.wikipedia.org/wiki/Cryptomnesia 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.

!!This episode contains examples of the following tropes:

* AdaptationalModesty: The original tale by Brown had the human protagonist running around in the nude. Kirk only suffers some ClothingDamage from his peril.
* BambooTechnology: Kirk's cannon.
* BigWhy: 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!''
* BloodSport: Kirk accuses the Metrons of this, declaring "You'll have to get your entertainment somewhere else" after refusing to kill the defeated Gorn.
* CaptainsLog: The Metrons are kind enough to provide Kirk with a recorder for this purpose.
* ClothingDamage: Kirk tears his pants (in a totally non-FanService way) to make a fuse for his [[LockingMacGyverInTheStoreCupboard cannon]].
* CrazyEnoughToWork: Kirk's makeshift cannon. Future ''Franchise/StarTrek'' media would end up repeatedly mentioning that Kirk got ''really'' lucky that the cannon didn't just blow up and kill him.
* DangerousForbiddenTechnique: 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.
* DisproportionateRetribution: 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, meaning that they decided to kill everyone at the outpost without trying to communicate with them and knowingly refused their attempt to surrender.
* EarlyInstallmentWeirdness:
** 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."
* ForcedPrizeFight: The plot.
* GadgeteerGenius: Kirk shows hints of this when he cobbles together a crude cannon out of the elements in the landscape.
* GeorgeLucasAlteredVersion: The remastered version includes a few shots of the Gorn blinking.
* HumansAreSpecial: The Metrons' justification for sparing the ''Enterprise.''
* InsultBackfire: 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!
* ItCanThink: 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.''
* TheJuggernaut: The Gorn.
* LateToTheTragedy: The outpost was destroyed at least a day before the ''Enterprise's'' arrival.
* LizardFolk[=/=]TheReptilians: The Gorn again.
* LockingMacGyverInTheStoreCupboard: 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.
* TheMainCharactersDoEverything: 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.
* MightyGlacier: The Gorn may be much stronger than Kirk, but is also much slower.
* MoralMyopia: 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 [[DoubleStandard killing is bad]].
* NoNameGiven: The Gorn Captain's name is never mentioned. Some Expanded Universe sources gave him different names: either [[IHaveManyNames S'alath, S'slee or S'salk]].
* OnlySmartPeopleMayPass: 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).
* OtherworldlyAndSexuallyAmbiguous: The Metron who talks to Kirk at the end has an androgynous appearance[[note]]and is one of several aliens in the series where the body and voice are provided by actors of different genders[[/note]] and wears a shimmering dress-like garment.
* PunchPunchPunchUhOh: Kirk lands some pretty good blows on the Gorn, but he's less effective than [[GoodOldFisticuffs usual]].
* Really700YearsOld: The Metron 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)''
* ReptilesAreAbhorrent: 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.
* ScifiWritersHaveNoSenseOfScale: 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.
* SssssnakeTalk: The Gorn says "I weary of the chasssse. Wait for me. [[YouWillBeSpared I ssshall be]] [[MercyKill mercccciful and quick.]]"
* SheatheYourSword: Kirk refuses to kill the Gorn.
* SpiderSense: 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.
* SwordOverHead: Kirk, in this position with the Gorn, refuses to kill him.
* ViewersAreMorons: Wait... what's the hardest substance known to man?
* WorthlessYellowRocks: 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.
* WowingCthulhu: 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.
* YouCalledMeXItMustBeSerious:
-->'''Spock:''' Destruction of the alien vessel will not help that colony, Jim.