The second pilot for the Original Series, produced after NBC rejected [[Recap/StarTrekS1E0TheCage "The Cage"]].
Kirk, Scotty, and Sulu were introduced here, along with Spock as we know him (he was in "The Cage", but [[EarlyInstallmentWeirdness with some different personality traits]]). [=McCoy=] and Uhura were introduced in the first regular episode, [[Recap/StarTrekS1E1TheManTrap "The Man Trap"]], along with Yeoman Rand, while Chekov didn't show up until season two (though ''Film/StarTrekIITheWrathOfKhan'' implied that he was somewhere on the ''Enterprise'' as early as [[Recap/StarTrekS1E22SpaceSeed "Space Seed"]]).
The ''USS Enterprise'', near the edge of the known galaxy, encounters an old-style ship recorder from the ''SS Valiant,'' a ship that disappeared 200 years ago in the same mission to penetrate the "galactic barrier" as the ''Enterprise.'' Spock interprets the recorder's message which indicates that the captain of the ''Valiant'' ordered a self-destruct on the ship after trying to access information on humans with extrasensory perception (ESP). The ''Enterprise's'' newly-arrived psychiatrist, Elizabeth Dehner, comments on the abilities of humans with high ESP ratings - abilities which don't normally go beyond mild precognition. Forewarned, the crew prepare to encounter a magnetic storm at the edge of the galaxy.
The ''Enterprise'' is crippled by a mysterious electric field, damaging its engines, killing nine crewmen and shocking both Dehner and Lieutenant Commander Gary Mitchell. Mitchell is taken to the infirmary where he has developed [[GlowingEyesOfDoom strange silver eyes.]] Being an old Academy friend of Kirk, the captain personally attends to Mitchell only to find that he has been speed-reading through the ship's library at the blink of an eye. Kirk and Spock consult the ship's records to find that Dehner and Mitchell scored the highest ESP ratings. Mitchell slowly begins to develop strange powers such as reading minds, telekinetic skills and clairvoyance. While Dehner is taken in by Mitchell's transformation, Spock recognizes that Mitchell is becoming colder, more ruthless and will develop powers beyond their imagination. Spock recommends that the ''Enterprise'' head to the planet Delta Vega where a lithium-cracking station could help repair the ship's engines. Bereft of humans, Mitchell could be marooned there. Either that or execute him. Kirk, torn between his old friend and what he has become, struggles with the decision.
Mitchell becomes more confident with his abilities as the ''Enterprise'' reach Delta Vega, commenting that he could read Kirk and Spock's minds, knowing what they have planned. Nevertheless, they succeed in incapacitating him for a moment and bringing him to Delta Vega, keeping him in a forcefield cell. Kirk continues to struggle with what to do with Mitchell, even as he becomes stronger while Dehner continues to speak of the possibilities of a new form of human. While the ''Enterprise'' is repaired, Mitchell escapes and kills [[RedShirt an engineer]] and incapacitates Kirk and Spock. He takes Dehner with her to the hills of Delta Vega, revealing that she too has developed the silver eyes, and therefore will have the same abilities.
Kirk is woken by the chief medical officer and goes after the pair. Mitchell demonstrates his powers to Dehner by creating a garden from nothing and remarks on what the two of them could do. He senses Kirk approaching and sends Dehner to meet him. Kirk and Dehner have an argument on what Mitchell has become. Dehner tries to defend Mitchell but Kirk appeals to the psychiatrist in her to recognize what Mitchell has become. Mitchell himself appears and easily brings Kirk down. Dehner sees how Mitchell has become inhuman and attacks him with her own powers. He retaliates and deals her a blow but not before his power is sapped and Kirk attacks. A rather lame fistfight ensues and ends with Mitchell buried in the grave he had prepared for Kirk. Dehner shows remorse for her actions in her dying breath as she passes, leaving Kirk alone to return to the Enterprise.
''"Captain's log, stardate 1313.8. Add to official losses, Dr. Elizabeth Dehner. Be it noted she gave her life in performance of her duty. Lieutenant Commander Gary Mitchell. Same notation(...)He didn't ask for what happened to him."'' notes Kirk, prompting Spock to say that he felt sympathy for Mitchell too. Grimly amused, Kirk comments ''"I believe there's some hope for you after all, Mr. Spock."''
* AGodAmI: Gary Mitchell rather handily proclaims himself.
* ApocalypticLog: The recorder left by the ''SS Valiant.''
* BewareTheSuperman: Gary Mitchell.
-->'''Kirk''': You were a psychiatrist once. You know the ugly, savage things we all keep buried, that none of us dare expose. But he'll dare. Who's to stop him?
* ClothingDamage: Of course Kirk's first fistfight of the show would get his shirt ripped.
* CriticalHesitationBlunder: Subverted. Kirk is fighting Gary Mitchell while Mitchell's psionic powers are temporarily suppressed. As Kirk is about to smash in Mitchell's head with a rock, he hesitates for a moment as he says "Gary, forgive me." Because of the delay Mitchell regains his powers and Kirk loses his chance to kill him. A few seconds later Kirk gets lucky and manages to kill Mitchell anyway.
* DrunkWithPower: Initially Mitchell doesn't seem all that different, but as his power grows he becomes more aloof and arrogant.
* EarlyInstallmentWeirdness: Given that this was the second pilot, it really shows, from the less-detailed uniforms (and no red shirts!) to the lack of notable characters like Uhura and Bones.
** Initially there was no 'Space the final frontier' narration. This has been 'corrected' in the remastered version.
** Sulu is present but is not the helm officer (he is the ship's botanist).
** Kirk's tombstone gives his middle initial as "R" rather than "T." One of the novels explains this as an inside joke from Kirk and Gary's time together at the academy.
** The make-up on Spock is noticeably different from the rest of the show, giving him a somewhat comical appearance if you're comparing it to his "normal" look. He also wears command yellow rather than science blue.
** Spock also does a lot of shouting, in contrast with his later established calm deadpan.
** Spock mentions that "one of [his] ancestors married a human female", which seems an odd way of putting it in retrospect, since it was established only an episode or two later that it was his father and mother.
** The "Materializer" (Transporter) is the Helm controls from the Bridge.
** The ship needs ''lithium'' crystals to re-energize the ship, rather than ''di''lithium.
** Kirk is wearing only two rows of rank insignia.
*** Everyone else is wearing zero or one, and they look pretty shoddily attached.
* EvilGloating: When Kirk and Spock come to take Mitchell into custody for the trip to Delta Vega, he fends them off with his telekinetic powers, then starts monologuing about how much superior he now is. He gets so caught up in it that he stops paying attention to his surroundings long enough for Kirk to get the drop on him.
* ExplosiveInstrumentation: This being only the ([[Recap/StarTrekS1E0TheCage second]]) pilot episode of Star Trek, it marks the series debut of this much-worn trope, as the Enterprise's flight through the great barrier causes every single console on the bridge to start sparking or going up in flames. Justified somewhat on this occasion by the unknown properties of the barrier frying most of the ship's systems.
* FreezeFrameBonus: As Kirk and Spock are reviewing Mitchell's ESP record, it mentions an incident on Deneb IV where Mitchell held a psychic conversation with one of the natives. Later, Kirk and Mitchell mention the same incident, but Gary mentioning ''less'' after-effects, "except for the eyes."
* KirkSummation: Directed at Dehner rather than Mitchell, who has pretty much gone off the deep end. Kirk reminds the doctor that for all his vaunted powers, Mitchell is still human, subject to human frailties.
* KneelBeforeZod: Mitchell demands Kirk to pray to him and then ''forces'' him to kneel.
* MindOverMatter: Mitchell is able to tinker with ship controls from his sickbed, and later uses his telekinesis to strangle Kelso with a cable.
* MysteriousMiddleInitial: Doubly mysterious, considering it's the last time we'll see ''this'' middle initial for Kirk.
* PrematurelyMarkedGrave: Commander Gary Mitchell has gained tremendous psionic powers, threatening the Enterprise and its crew. When Captain Kirk tracks him down and confronts him, Mitchell causes a grave and a headstone to appear. The headstone has Kirk's name and the stardates of his birth and (anticipated) death.
* ProHumanTranshuman: Dehner, who sides with Kirk (and humanity) in the end. Her FamousLastWords say it all.
-->'''Elizabeth Dehner''': "I'm sorry...you can't know what it's like...to be almost a god."
* ReCut: The original edit submitted to NBC (included on the Blu-ray set) was a bit different. It opened with different music and a different narration by Shatner. The teaser was after the opening credits, and once they went to RedAlert we saw scenes of the characters rushing through the halls along with their actor credits. The act breaks had Quinn-Martin-style "Star Trek: Act II" cards. When the show went into production, the pilot was edited to match the style of the main series.
* RedRightHand: Mitchell's silver eyes, denoting godlike powers. Also, his hair slowly becomes gray, starting with his sideburns.
* [[SwordOverHead Rock Over Head]] Kirk hesitates to smash Gary's head in with a huge rock. This was not a good idea
* SacrificialLamb: The obvious one is Gary Mitchell himself, as he's introduced as being Kirk's closest and oldest friend, but he's KilledOffForReal by the end of the episode. More broadly, however... the ''entire cast'' were talked about in the network sales brochures and the like as if they were all going to be regular characters if the show was green-lit, so unexpectedly killing off Lee Kelso two-thirds of the way through the episode might also count (as we might otherwise have been led to believe he's going to survive and remain the show's navigator).
* SadisticChoice: Kirk can kill his friend now, or abandon him on an uninhabited planet.
* ScifiWritersHaveNoSenseOfScale: The Enterprise goes out to explore the very edge of the galaxy, and its main engines are damaged, but it's okay because there's a planet a few light-days away with a mining station where they can make repairs. In cosmic terms, this is like driving out into the uncharted wilderness and breaking down right outside a gas station. (A light-day is approximately the diameter of the solar system; even in the denser parts of the galaxy, far in from the edge, stars are typically light-''years'' apart.)
* SmartPeoplePlayChess: The episode begins with Kirk and Spock playing three-dimensional chess. Spock comments that Kirk's style of playing is irritating.
* SuddenNameChange: Crewman Mitchell, possessed of near-omnipotent alien powers, fights Kirk and creates an open grave with a tombstone reading "James '''R.''' Kirk". This would normally be a minor matter but given how many times Kirk later introduces himself as "James '''T.''' Kirk" it's actually quite jarring.
* SuddenlyShouting \ SayMyName
** '''KIRK:''' Did you hear him joke about compassion? ABOVE ALL ELSE, A GOD NEEDS COMPASSION, MITCHELL!!
* TemptingApple: When Mitchell is attempting to persuade Dehner of his viewpoint that they're above humanity now, he summons a [[SpaceX Kaferian apple]] tree out of nowhere and splits an apple with her.
* WhamShot: The shot of Dehner with silver eyes.
* WithGreatPowerComesGreatInsanity: The scope of Mitchell's abilities pretty much drive him loony.
* WizardDuel: Mitchell vs Dehner, complete with ShockAndAwe.