[[quoteright:300:https://static.tvtropes.org/pmwiki/pub/images/the_enemy_within.png]]
[[caption-width-right:300:[[ChewingTheScenery If you look close, you can see bits of scenery in his teeth.]]]]

'''Original air date:''' October 6, 1966

Kirk, Sulu and some unusually long lived RedShirts are collecting specimens from Alfa 177 when someone falls down a hill and gets a boo-boo. He's beamed up, along with a strange, magnetic ore that he picked up in his fall. Shortly after, Captain Kirk beams up, feeling a little woozy. Scotty leads him away, even as Kirk warns him not to leave the transporter room unattended. What beams up next is... Captain Kirk! You can tell it's an evil Kirk! He's wearing eye liner and sweating like a pig!

So now, Kirk's got a doppelganger running about acting more like Captain Morgan than Captain Kirk. No one dares use the transporter now, so Sulu and company are left behind on a planet that gets well below 0 at night. And it seems that whatever happened to Kirk also happened to a little canine creature. One's a sweet little lap dog, the other... well, ever see that WesternAnimation/TomAndJerry cartoon called "WesternAnimation/TheCatsMeOuch"?
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!!The Tropes Within:
* AttemptedRape: Poor Janice is subjected to this by Evil Kirk.
* BadassInDistress: Sulu is trapped on the planet during its cold night and nearly freezes to death.
* BalanceBetweenGoodAndEvil: [[AnAesop The aesop]] is that a person can't function without both the light and dark sides. Without the light, Evil Kirk is unrestrained and violent; without the darkness, Good Kirk is impotent.
-->'''Spock:''' And what is it that makes one man an exceptional leader? We see here indications that it's his negative side which makes him strong, that his evil side, if you will, properly controlled and disciplined, is vital to his strength.\\
\\
'''[=McCoy=]:''' ''(to Good Kirk)'' We all have our darker side. We need it! It's half of what we are. It's not really ugly. It's Human.[...] The intelligence, the logic. It appears your half has most of that. And perhaps that's where man's essential courage comes from.
* BravingTheBlizzard: Mr. Sulu and three others get trapped on the planet the ''Enterprise'' is orbiting. As night comes on, the wind kicks up and the temperature falls dramatically. Due to the malfunction, any heaters transported down are rendered useless, and the landing party is forced to use their phasers to superheat rocks in order to stay warm enough to survive.
* ClearMyName: Certainly important to Good Kirk.
* ColorCodedForYourConvenience: Evil Kirk wears the yellow command uniform. Good Kirk wears his casual green shirt. That is, until Evil Kirk realizes he can (temporarily) fool people by changing his shirt.
* DirtyCoward: Evil Kirk lacks Good Kirk's courage (see VillainousBreakdown).
* EarlyInstallmentWeirdness:
** It's a bit weird to see Spock call himself "half human, half alien," when for the bulk of the series he clearly identifies as Vulcan.
** Sulu and the rest of the exploration team could have been brought aboard had they thought to use the shuttlecraft. The only reason they didn't use it is because they hadn't built the shuttlecraft sets and models yet.
** As with a few of the other early episodes, several of Kirk's captains logs are done in the past tense rather than present tense.
* EnemyWithin: Well, duh! Look at the episode's title!
* EnemyWithout: How it comes to be.
* EverybodyLives: The only fatality is the cute but non-sentient dog-unicorn thing from the planet.
* EvilIsHammy: Good ''Lord''. Evil Kirk makes all of Shatner's other performances look subdued.
-->'''Evil Kirk:''' I'm Captain Kirk! I'm Captain Kirk! I'M CAPTAIN KIRK! '''IIIII'M CAPTAIN KIRRRRRK!!!'''
* EvilMeScaresMe: "I've seen a part of myself no man should see."
* EvilTwin: Created by transporter accident. This is a rare case where the evil twin isn't an imposter, but actually ''is'' the ''real'' person (albeit the negative, disagreeable side of that person).
* FirstNameBasis: Evil Kirk tells Yeoman Rand to call him Jim, as he calls her Janice.
* FromBadToWorse: While Scott is still trying to find out why the transporter split Kirk in two, Evil!Kirk blasts some of the circuitry with a phaser. After discovering this, Scott predicts he will need and entire week to fix it.
* GallowsHumor: Sulu cracks a few jokes about just needing a coffee when it's a very real possibility they could die from cold.
* GoodEyesEvilEyes: Evil Kirk wears eyeliner and his eyes are either narrow and shifty, or wide and crazy.
* GotOverRapeInstantly: Evil Kirk almost succeeds in raping Yeoman Rand. While she's traumatized for the rest of the episode, she's back to mooning over Kirk in her next appearance as though nothing had happened. You'd think the very sight of him would make her want to transfer to another ship, but I guess not.
* {{Guyliner}}: Evil Kirk wears it, as per the ExcessiveEvilEyeshadow trope. He also puts on some make-up in attempt to hide the scratches Rand put on him.
* HelplessGoodSide: Evil Kirk got Kirk's willpower, determination, and aggression, leaving Good Kirk rather meek and indecisive. It's played with in that Good Kirk also got the original Kirk's ''courage'', while despite all his agression Evil Kirk is at heart a coward.
* HesBack: Kirk's halves are put back together and it's obvious he's survived the process, but the real "He's Back!" moment comes when he decisively says "Get those men aboard fast!"
* HesDeadJim: For the [[TropeMaker first time]] in the series! After they attempt the re-merging with the creature, [=McCoy=] and Spock examine the limp form that beams up.
-->'''Spock''': It seems the shock was too much.\\
'''[=McCoy=]''': He's dead, Jim.
* IdenticalTwinIDTag: Apart from wearing a different shirt, Evil!Kirk is distinguished from Good!Kirk after Rand scratched his face. This breaks down when Evil!Kirk changes shirts, puts makeup on his face, and scratches Good!Kirk in turn.
* INeedAFreakingDrink:
** Evil!Kirk demands Saurean brandy when he enters sick bay. Later, Bones decides that both he and Good!Kirk need it.
** Sulu cracks a few jokes about needing coffee (due to the freezing temperatures of the planet), then says that ''sake'' will do as well.
* IdiotBall: Who exactly thought it was a good idea to have Yeoman Rand questioned about her attempted rape by the accused perpetrator? The same people who thought it was a good idea to let the accused walk around after being identified by an eyewitness. Though Good Kirk pretty quickly proved it wasn't him, he has no scratches on his face. This being ''Star Trek'', everyone is immediately convinced that something substantially weirder is going on.
* JekyllAndHyde: A transporter malfunction splits Kirk's good and evil sides.
* LiteralSplitPersonality: Kirk of this episode is a famous example. It is learned that the "evil" side proved to be the side with the strength to make tough decisions, proving that both halves are needed for the whole to work. And the seemingly meek, passive "good" side turns out to have more courage than the other, whose angry defiance hides terror of losing independent existence.
-->"Can half a man live?"
* MundaneUtility: Sulu uses a phaser to heat up some rocks for warmth. Beginning a long, proud ''Trek'' tradition, fully in fitting with its themes, that phasers are as much tools as weapons.
* MustHaveCaffeine: Sulu asks if it's possible for them to send down a pot of coffee. Or possibly, he just wants something hot. [[INeedAFreakingDrink He would've settled for sake.]]
* NegativesAsAPositive: After a transporter accident splits Kirk into two people, one good and one evil, it's revealed that his good side isn't capable of command. Spock postulates that it is humanity's faults, tempered by their morals and ethics, that give them the ability to lead.
* OOCIsSeriousBusiness: Never has it been more serious! And on both sides: Good Kirk lacks the command presence and decisiveness he's previously demonstrated and which is necessary for TheCaptain, Evil Kirk lacks empathy, courage, or any real impulse control. The trick is not to figure out which Kirk is which, but that they're ''both'' equally not "real" Kirk.
* OutOfCharacterAlert: Kirk is split into his good and evil sides by a transporter accident, which leaves several of the Enterprise crew stranded on a hostile planet. Spock feels a bad vibe when the evil Kirk, passing himself as the good one, walks onto the bridge. Then he declares that the men can't be saved and orders the ship to leave orbit, at which point Spock knows it's not the real one - since when would [[AFatherToHisMen James T. Kirk]] leave his crew to die?
* PlotHole:
** Why doesn't Kirk send a shuttlecraft to collect the stranded men? (The [[WatsonianVersusDoylist Doylist]] answer is because they hadn't finished building the shuttlecraft set.)
** Why doesn't the ''Enterprise'' crew beam down some blankets? It's not like bedding can get split into Good and Evil. The heaters that were beamed down didn't work, but this shouldn't apply to blankets. Or coffee, for that matter.
** They could have even beamed the characters up and confined the resulting duplicates until the problem was fixed.
* ThePowerOfHate: When Kirk gets split into Good Kirk and Evil Kirk, Good Kirk is barely able to function because, as Bones points out, he needs the power of hate as well as love.
* RidiculouslyCuteCritter: Sulu finds something that looks like a mash up of a cocker spaniel, a unicorn and a baby dragon with some screwy looking antenna. And it's flippin' adorable! (When its evil half isn't trying to eat your face, that is.) What a pity that it dies in the end.
* SarcasmFailure: See GallowsHumor. As the (admittedly improbable) cold gets to the stranded crew members, Mr. Sulu's last communication to the ''Enterprise'' is laconic and quiet.
* ShadowArchetype: The good-yet-indecisive Kirk and his evil-yet-effective twin: "I have to take him back... inside myself. I can't survive without him. I don't want him back. He's like an animal, a thoughtless, brutal animal -- and yet it's me. Me." Really though, both the good and evil Kirk are Shadow Archetypes to the real Kirk; he wouldn't want to be either one of them.
* ShirtlessScene: Shortly after beaming up, Good Kirk feels a sudden urge to go change his shirt. The ladies (and Mr. Spock) get some eye candy, and there's a reason for the sudden ColorCodedForYourConvenience.
* SickCaptiveScam: Evil!Kirk feigns weakness so that Good!Kirk undoes his restraints in Sickbay and assists him to get up and walk to the Transporter Room, then overpowers him and assumes command.
* {{Slurpasaur}}: The alien creature Sulu finds is simply a cocker spaniel or terrier wearing a horn, antennae, and a thick coat and long rat-like tail.
* TakeAThirdOption: Spock explains how he uses this to live. His human and Vulcan natures are constantly in conflict, but he uses his intelligence to keep both in line and make them work together.
* TantrumThrowing: Evil Kirk throws a fit when he can't get his way.
* TapOnTheHead: A legendary subversion. When good Kirk is confronted by a phaser-wielding evil Kirk, Spock steps around a corner behind evil Kirk and knocks him out. The script called for Spock to do this by hitting evil Kirk with the butt of his phaser pistol. Creator/LeonardNimoy thought that was inappropriate for a Vulcan -- it's ''so'' uncivilized -- so he and the director improvised the very first use of [[PressurePoint the Vulcan nerve pinch]]. Nimoy demonstrated it on Shatner, and Nimoy credited Shatner with selling it.
* {{Technobabble}}: Spock's explanation of how the transporter was repaired comes off as this.
* {{Understatement}}: Before beaming up, Kirk tells Sulu that the temperature on the planet gets down to some [[ArtisticLicensePhysics ridiculously low arbitrary temperature]] at night, [[RealityIsUnrealistic similar to the dark side of the Moon]]. Sulu replies, "That's nippy."
* VillainousBreakdown: Evil Kirk finally gives up when he breaks down, sobbing, "I don't wanna die!"
-->"I don't wanna go back. I wanna LIIIIIIVE!!!!"\\
"[=We...WILLive!....BOTHuvus=]!"
* WarringNatures: Spock claims that his human side and his "alien" side are constantly doing this.
* WhatTheHellHero: [=McCoy=] gives this to Spock when he uses Kirk's situation to coolly analyze the roles of good and evil in human nature. Spock doesn't back away from the accusation, but does offer an apology of sorts to Kirk for his seeming to be uncaring.
* WholePlotReference: Creator/RichardMatheson's main influence on writing this episode was ''Literature/TheStrangeCaseOfDrJekyllAndMrHyde'', as he envisioned the story put in a science fiction context. He eventually came up with the idea of the transporter causing a man to be split into two halves.
* WithAllDueRespect: Spock tells Kirk that he doesn't have the luxury of indulging his conscience:
-->Captain, no disrespect intended, but you must surely realise you can't announce the full truth to the crew. You’re the Captain of this ship. You haven’t the right to be vulnerable in the eyes of the crew. You can’t afford the luxury of being anything less than perfect. If you do, they lose faith, and you lose command.
* WritersCannotDoMath: No matter what scale you use, the temperatures given for the planet's surface mean Sulu's team would have frozen to death before the episode is even halfway over.
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