The Enemy Within (episode)
A transporter malfunction splits Captain Kirk into two people – one good and the other evil, and neither capable of functioning well separately.

Summary

Teaser
During a survey of Alfa 177, geological technician  slips down a rock, gashing himself badly and smearing his uniform with a strange magnetic type of yellow ore. He beams up to the  for treatment.

Detecting a curious overload in the transporter circuitry, Lieutenant Commander Montgomery Scott has Fisher decontaminated before reporting to sickbay, but the problems have already begun; the strange ore has altered the function of the transporter. 

Next, Captain Kirk beams up from the planet, before the fault is discovered. He apparently materializes normally and Scott escorts the disoriented captain out of the room. Kirk is, in fact, a shadow of himself. Due to this transporter accident, Kirk has been split into two beings. The first that materialized embodies all of Kirk's positive qualities. Moments later, after Kirk and Scott have left, Kirk's evil twin — Negative Kirk — materializes in the transporter chamber.

Act One

Some time passes before the mishap is discovered. Negative Kirk demands Saurian brandy from McCoy in sickbay and proceeds to roam the ship's corridors drunk. McCoy goes to Commander Spock about this who then goes to Kirk about what the good doctor tells him. Kirk shrugs it off telling Commander Spock that McCoy was just pulling his leg. Negative Kirk, who is now instinctively consumed by lust for his beautiful Yeoman – Janice Rand – is alone with Rand in her quarters, drunk and amorous. Negative Kirk mentions to her the feelings they've been hiding, claiming she is "too beautiful to ignore," and "too much woman," and that they've both been "pretending too long." Negative Kirk suddenly grabs Rand and shouts,"Let's stop pretending!" He pulls her in close, put his arms around her, and mutters, "…Don't fight me, Janice." He then starts kissing her very aggressively, and as she is forcefully trying to fight back, Negative Kirk pushes her to the floor and attempts to rape her. But she defends herself and leaves a large scratch on Negative Kirk's face. During the struggle, Negative Kirk attacks Crewman Fisher, who was walking by Rand's quarters and saw the attack. In sickbay, a crying and flustered Rand tells Kirk, plus Spock and McCoy, that the Captain tried to assault her, in which both she and Fisher accuse him of this. Kirk firmly denies this, whereupon Spock deduces that there is an impostor of him aboard the Enterprise.

Act Two

Scotty finds that the yellow ore Fisher beamed up with somehow caused an overload in the transporter. The transporter does indeed work but they dare not use it for risk of duplicating Sulu and the rest of the landing party. Kirk tells Spock that he must inform the crew of what has happened to him, since they deserve to know. Spock, with all due respect, tells Kirk that as he is the captain, he cannot afford to be anything less than perfect in the eyes of the crew. If he does appear so, the crew will lose faith in him – and in turn, he will lose command of the Enterprise. Kirk knows this and wonders why he just forgot it just now. Later, on the bridge, Kirk makes an announcement to his crew from his chair about the impostor aboard. While making the announcement, Negative Kirk is rummaging through the captain's quarters. Kirk informs the crew that the impostor can be identified by scratches on his face, and warns the crew to set the phasers for stun and must not injure the impostor. Negative Kirk angrily destroys the captain's desktop monitor and rants at the top of his lungs "I'm Captain Kirk… I'M CAPTAIN KIRK!!!"

Negative Kirk goes to Kirk's mirror and finds make-up on the table. He applies some of it to his scratches and they are now barely visible. He opens the door to Kirk's quarters and finds Crewman  walking down the corridor near the room. Negative Kirk asks Wilson for his phaser; Wilson hands it over and is promptly knocked out. Later, both Kirk and Spock in the briefing room try to figure out where Kirk would go on the Enterprise to elude a mass search. Kirk quickly deduces that Negative Kirk is hiding in the lower levels of the ship – the engineering deck. He and Spock head there. In main engineering, a cat and mouse game ensues between the two Kirks and they confront each other near the warp core. Just as Negative Kirk is about to kill Kirk, Spock knocks him out with the Vulcan nerve pinch, but not before Negative Kirk's phaser accidentally discharges and a shot disables the transporter ionizer, making it harder to rescue Sulu and the landing party who are trapped on the rapidly freezing planet.

Act Three
Meanwhile, on the planet below, the remaining landing party is suffering through the increasingly bitter cold. Attempts to beam heaters and other support devices produce only non-functional duplicates. Kirk speaks to Sulu in the Enterprises briefing room, trying to reassure his helmsman, all the while growing more and more unsure of his command abilities. Spock cuts in and tells Sulu to hold on for just a little while longer.

Negative Kirk is screaming while being restrained on a bio-bed in sickbay, in pain from his body functions having been weakened from the duplication process. Kirk takes Negative Kirk's hand, tells him not to be afraid and to use his mind, rather than his savagery. McCoy takes Kirk aside for a brandy. Kirk realizes through McCoy that he needs his negative side of himself back but does not want it back. McCoy assures his captain that all Humans have a dark side to them and that his strength of command lies in his negative self.

Finally, Scott and Spock believe they have isolated and repaired all the damage. Spock contacts Kirk and asks him to come down to the transporter room. An Alfa 177 canine test animal, previously split, is sent through to see if it will reintegrate. Spock and Scott subdue the fierce canine with a hypospray and place it beside its good self on the transporter pad. Spock and Fisher place the two canines on the transporter pad and Scotty energizes. "If this doesn't work, I don't know what will," Scotty says. The good and fierce canines become one again, but it rematerializes dead from the shock from having suddenly had its two halves reintegrated forcefully.

Act Four
Negative Kirk recovers in sickbay while Sulu contacts Kirk from the planet, just before he succumbs to the extreme cold. Kirk decides to release his evil half and have both of them go through the transporter, but Negative Kirk attacks and overpowers Kirk in sickbay and dresses in Kirk's wraparound tunic. Later, heading to the bridge, Negative Kirk, pretending to be the good Kirk, runs into Janice Rand outside a turbolift and explains to her that the transporter malfunctioned, and that the animal part of him was in her cabin with her during the attempted rape. Negative Kirk, again posing the good Kirk, also points out that the evil Kirk scratched his face to make them more alike. Arriving on the bridge, Negative Kirk orders navigator Lieutenant John Farrell to take the ship out of orbit and abandon Sulu along with the landing party, callously asserting that they cannot be saved. At this point, the good Kirk appears on the bridge with McCoy. At first, Farrell and the others are confused as to which one is the good Kirk, but soon Negative Kirk collapses under the strain – screaming and crying "I want to LIVE!" before falling into Kirk's arms.

Later, both Kirks are taken to the transporter room to be reintegrated as one being. Spock handles the transporter console and promises Kirk that he will take command of the Enterprise if the procedure is unsuccessful. Spock energizes the transporter and the two Kirks disappear. After a few tense moments, Spock materializes Kirk back in the transporter chamber as one person. To Spock and McCoy's relief, the much more confident Captain Kirk steps off the pad and orders that the landing party be rescued immediately. Sulu and the others are beamed back aboard, frostbitten but alive.

On the bridge, Rand awkwardly tries to explain to Captain Kirk what his impostor told her about what had happened and when she tries to elaborate further, Kirk simply thanks her and walks away. When Rand hands a PADD to Spock to sign, the first officer says to her, "The, uh, impostor had some interesting qualities, wouldn't you say, yeoman?" She takes the PADD and stylus from him and walks away in a huff as Kirk orders that the Enterprise lift out of Alfa 177's orbit to continue its mission.

Log entries
 Captain's log, USS Enterprise (NCC-1701), 2266

Memorable quotes

"Oh! Captain! You startled me! Is there something that you…?"
"Can I help you, Captain?"
"Jim, will do here, Janice."
"Ooohhh…"
"You're too beautiful to ignore… Too much woman."
"We've both been… pretending too long."
 - Negative Kirk', before attacking Janice Rand

"You can't afford the luxury of being anything less than perfect. If you do, they lose faith, and you lose command."
 - Spock, to Kirk's good duplicate 

"I'm Captain Kirk. I'M CAPTAIN KIRK!!!"
 - Negative Kirk

"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."
 - Spock, to McCoy

"If I seem insensitive to what you're going through, captain, understand: it's the way I am."
 - Spock to Kirks good duplicate, as the latter begins to lose his will to command as a result of the transporter accident

"Do you think you might be able to find a long rope somewhere and lower us down a pot of hot coffee?"
"I'll see what we can do."
"Rice wine will do if you're short on coffee."
 - Sulu and Kirk's good duplicate 

"Any possibility of getting us back aboard before the skiing season opens down here?"
 - Sulu 

"He's like an animal. A thoughtless, brutal animal. And yet it's me. Me!" 
 - Kirks good duplicate, on his evil counterpart

"We all have our darker side. We need it! It's half of what we are. It's not really ugly. It's Human."
 - McCoy, to Kirk's good duplicate 

"The intelligence, the logic. It appears your half has most of that. And perhaps that's where man's essential courage comes from."
 - McCoy, to Kirk's good duplicate

"He's dead, Jim."
 - McCoy, on the test animal 

"Being split in two halves is no theory with me, doctor. I have a Human half, you see, as well as an alien half, submerged, constantly at war with each other. Personal experience, doctor. I survive it because my intelligence wins over both, makes them live together."
 - Spock, on merging the two Kirks

"Janice! Hello. The animal part of me came to your cabin. He even scratched me to make us look more alike. I'd like a chance to explain it to you. You don't mind if I come to your cabin later?"
"No, sir."
 - Negative Kirk, encountering Janice Rand again by pretending to be his good counterpart
 

"Can half a man live?"
 - Kirks good duplicate, to his evil counterpart on the bridge 

"I want to live!"
 - Negative Kirk

"I've seen a part of myself no man should ever see."
 - Kirk to McCoy, after the successful reintegration

"The… impostor had some… interesting qualities, wouldn't you say, yeoman?"
 - Spock, further victimizing Rand

Background information

Production timeline
 Story outline by Richard Matheson: 
 Revised story outline: 
 First draft teleplay by Matheson: 
 Revised first draft teleplay: 
 Second draft teleplay: 
 Revised teleplay by John D.F. Black: 
 Final draft teleplay by Gene Roddenberry: 
 Additional revisions: , 
 Filmed:  – 
 Day 1 – , Tuesday – Desilu Stage 10: Ext. Alfa 177 surface; Desilu Stage 9: Int. Transporter room
 Day 2 – , Wednesday – Desilu Stage 9: Int. Transporter room
 Day 3 – , Thursday – Desilu Stage 9: Int. Kirk's quarters, Bridge
 Day 4 – , Friday – Desilu Stage 9: Int. Sickbay, McCoy's office
 Day 5 – , Monday – Desilu Stage 9: Int. Sickbay
 Day 6 – , Tuesday – Desilu Stage 9: Int. Engineering, Corridors, Janice Rand's quarters
 Day 7 – , Wednesday (Half Day) – Desilu Stage 9: Int. Corridors, Briefing room
 Score recording: 
 Original airdate: 
 First UK airdate

Story and production 
 Writer Richard Matheson's main influence on writing this episode was Dr. Jekyll and Mr. Hyde, as he envisioned 's classic 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. 
 The subplot of Sulu and three other crewmembers stranded on the planet was not present in Matheson's original script, and was added in staff re-writes. Matheson did not like the idea, as he had an aversion to B-stories in general, believing they slowed stories down. He explained, "My script stayed entirely with Bill [Shatner] having this trouble of his two selves, on the ship [...] They added a whole subplot about people down on the planet, ready to freeze to death, because they have [a] transporter functioning problem [...] I stuck entirely with Bill." 
 The last two scenes of Act One are switched in order from what appears in the script. In the teleplay, Kirk and Spock learn about the assault of Janice in sickbay, then head to the transporter room, where they are faced with the discovery that the transporter is creating duplicates. The act ends with Scotty suggesting, "We don't dare beam up the landing party. If this should happen to a man…" and Kirk exclaiming, "Oh, my God!" In the episode itself, the sickbay scene follows the one in the transporter room, and the act ends with Spock declaring, "There's only one conclusion – we have an impostor aboard." Director Leo Penn was known to reorganize scenes when he deemed them to be more dramatic in a different order from what was scripted. 
 In the final draft and the revised final draft of this episode's script, McCoy mused that part of "the Human condition" was having "an enemy within."
 Grace Lee Whitney once recounted that, while shooting the scene when a distraught, tearful Janice Rand accuses Captain Kirk of trying to rape her, William Shatner slapped her across the face to get her to register the proper emotion.  As they shot the rape scene days earlier, Whitney couldn't get into the same emotion successfully, and it was Shatner's "solution" to the problem. (The Longest Trek: My Tour of the Galaxy, p. 94)

Props and sets 

 The scanning device used by Scott to scan the ore on Fisher's uniform appears to be a modified Nuclear-Chicago Model 2586 "Cutie Pie" radiation detector.  This Feinberger reappeared in , , and .

 A shot, showing two extras (Frank da Vinci and Ron Veto) in red technician jumpsuits (and Veto holding the aforementioned "Cutie Pie" prop) in the engineering set was filmed, but cut from the episode. It was probably filmed as an insert shot for scenes at engineering. 
 The gauzy, red-bordered triangular set piece behind which the evil Kirk emerges briefly in engineering during the hunt scene appears to have been left over from the early briefing room as seen in  and .
 The unit that Negative Kirk accidentally phasers in engineering was recycled as the housing for the main circulating pump for the PXK pergium reactor in .

Special effects 

 This is the only appearance of the showering phaser effect, used when Sulu heats the rock to provide warmth for himself and the other stranded on Alfa 177.
 There are two split-screen effects used: after Negative Kirk is neck-pinched by Spock; and in sickbay when Kirk takes the hand of his evil counterpart. All other instances of the two Kirks appearing in the same shot were achieved using stand-ins, who kept their faces turned away from camera: Eddie Paskey, who doubled as both Kirk and Negative Kirk; and stunt performer Don Eitner during the scene where Negative Kirk attacks Kirk in sickbay.

Costumes 
 At the start of the episode when Kirk is beamed up from Alfa 177, both he and his evil counterpart are missing the Enterprise insignia on their uniforms. Lieutenant Farrell is also missing his insignia at some points during the episode (and also in a shot recycled from this episode in ). The Star Trek Compendium (3rd ed., p. 35) suggests that the insignia were removed every time the uniforms were cleaned (union rules required them to be cleaned daily); and during production of this episode, someone forgot to replace the insignia on Kirk and Farrell's uniforms.
 The first season version of the captain's wraparound tunic was created for this episode, with the original purpose of differentiating Kirk from Negative Kirk. It reappears in  and , and in Kirk's briefcase in . The tunic was constructed of wool crêpe fabric, in contrast to the velour of the standard duty tunics.

Cast 
 Although Nichelle Nichols does not appear in this episode, her voice is heard on the intercom in several scenes.
 Frank da Vinci and Ron Veto together had a deleted scene in this episode.
 Edward Madden previously played an Enterprise geologist in .

Continuity 
 This episode marks the first time on screen that Kirk is duplicated in some form or fashion. This repeats again through , , and .
 Spock's log entry in this episode calls himself "Second Officer Spock", however, in "Court Martial" the computer lists him as "First Officer Spock" and from then on is referred to as a "first officer" until his return in  as simply science officer.
 This was the first episode to show the Vulcan nerve pinch, as well as the first time McCoy says "He's dead, Jim." Leonard Nimoy objected to the script's directive that Spock "kayoes" Negative Kirk on the head, so he improvised the neck pinch on the spot and demonstrated it on William Shatner for director Leo Penn. () The script's original directive survives to this day in the James Blish adaptation of this episode (in the book Star Trek 8) as Blish only had access to the scripts and not to the finished episodes.
 The transporter was depicted as the only mode of transport between a planet and the ship because this episode was written and filmed before the existence of the hangar deck and shuttlecraft were established, even though both of these locations are clearly built into the studio model of the Enterprise as seen in the episode.  As the lack of a shuttlecraft was almost immediately pointed out as a plot error, production and writing staff later stated that perhaps the shuttlebay was simply out of service for repairs during this episode, which would explain why the crew did not use shuttles to rescue their comrades (Star Trek Compendium)
 This episode establishes that the phaser two unit consists of a phaser one unit fitted into a pistol mount. Sulu can be seen doing this just before he phasers the rocks for warmth. This procedure is not seen in any other episode.
 In the sequence of aired episodes, this is the first episode where we see or hear the new middle initial for James Kirk. "Captain James T. Kirk" is briefly visible as Negative Kirk enters Kirk's quarters. The initial was first spoken in , but that episode aired after "The Enemy Within".
 In the latter part of the scene where the two Kirks appear together on the bridge, a close-up shot of the Negative Kirk shows the scratches on the right side of his face, although wider shots (and all earlier scenes) showed they were on the left side. This was due to the shot being reversed during editing. Director Leo Penn and cameraman Jerry Finnerman mistakenly filmed the close-up out of axis, breaking the 180-degree rule, and editor Fabien Tordjmann could only help by reversing it, hoping the audience won't notice the resulting continuity error. (These Are the Voyages: TOS Season One, 1st ed. p. 157)
 A reaction close-up shot of Spock on the bridge is recycled from . The same shot was used again in .

Apocrypha 
 In the novel Foul Deeds Will Rise, the duplication witnessed here is documented as 'the Alfa Effect', with another character duplicating the effect to give herself an alibi for various murders (One of her appears in public while the other commits the crimes), claiming to have adjusted the balance so that both duplicates share the same personality traits as opposed to the imbalance witnessed here (although the duplicates do demonstrate a divergence later, with one being more violent while the other accepts their failure).
 A cat version of "The Enemy Within" was featured in Jenny Parks' 2017 book Star Trek Cats.

Preview 
 The preview contains a Captain's Log recorded solely for the preview, but based on one from the finished episode: "Captain's log, stardate 1672.9.  Due to the malfunction of the ship's transporter, an unexplained duplicate of myself exists."

Reception 
 Actress Grace Lee Whitney was very unhappy about the last scene of this episode, in which Spock asks Yeoman Rand, if "the impostor had some very interesting qualities, wouldn't you say, yeoman?". In her autobiography, she wrote: "I can't imagine any more cruel and insensitive comment a man (or Vulcan) could make to a woman who has just been through a sexual assault! But then, some men really do think that women want to be raped. So the writer of the script (ostensibly Richard Matheson – although the line could have been added by Gene Roddenberry or an assistant scribe) gives us a leering Mr. Spock who suggests that Yeoman Rand enjoyed being raped and found the evil Kirk attractive!" (The Longest Trek: My Tour of the Galaxy, p. 95)
 Despite this, Whitney enjoyed this episode. "I liked 'The Enemy Within' because seeing Captain Kirk act in that fashion challenged me as an actress." ( #105, , p. 49) Several years later she echoed those sentiments, stating "I love "The Enemy Within" because it gave me a chance to really react and act with Bill Shatner. I love it! I loved the whole concept of him breaking into two characters because that really was what Kirk and Rand were about. There were two sides of Kirk and two sides of Rand. Rand was there to be of service to him but she was also in love with him. But she knew she mustn't go over-go the boundaries." (These Are the Voyages: TOS Season One, p. 159) 
 James Doohan stated that he thought William Shatner's performance in this episode was "pretty okay." (Beam Me Up, Scotty, p. 132)
 Director Leo Penn also praised Shatner's performance. "William Shatner's a very good actor and gave a very good performance… I had a good time on that show." (These Are the Voyages: TOS Season One, 1st ed. p. 159)
 Richard Matheson elaborated, "I thought Bill Shatner was brilliant. I loved what he did. He carried the whole thing. I was a little sorry that Roddenberry put so much emphasis on the crew being stuck on the planet [....] But I liked it and I was very satisfied with the production value." (These Are the Voyages: TOS Season One, 1st ed. p. 159)
 Roddenberry picked this as one of his ten favorite episodes for the franchise's 25th anniversary. (TV Guide, August 31, 1991)

Syndication cuts 
During the syndication run of Star Trek, the following scenes were typically cut from broadcast:

 Extended scene of Scott preparing to beam Kirk up to the ship.
 A longer conversation between Spock and Kirk about Negative Kirk demanding brandy from McCoy.
 Extended scene of Negative Kirk in Rand's quarters.
 Additional segments of discussion with Rand and Fisher in sickbay.
 Extra dialogue between Kirk and Spock, followed by another scene showing the two entering engineering to look for Negative Kirk.
 Longer reports from Sulu on the surface of the planet, including a split scene of Sulu speaking to Kirk, then a shot of the Enterprise in orbit, followed by Kirk's reply.

Video and DVD releases
 Original US Betamax/VHS release: 
 Original UK VHS release (two-episode tapes, CIC Video): Volume 3, catalog number VHR 2244, release date unknown
 US VHS release: 
 UK re-release (three-episode tapes, CIC Video): Volume 1.2, 
 Original US DVD release (single-disc): Volume 2, 
 As part of the TOS Season 1 DVD collection
 As part of the TOS Season 1 HD DVD collection
 As part of the Star Trek: Fan Collective - Alternate Realities collection
 As part of the TOS Season 1 Blu-ray collection

Links and references

Starring
 William Shatner as James T. Kirk

Also starring 
 Leonard Nimoy as "Mr. Spock"

Featuring
 DeForest Kelley as "Dr. McCoy"
 Grace Lee Whitney as "Yeoman Rand"
And
 George Takei as "Sulu"
 James Doohan as "Scott"
 Edward Madden as ""
 Garland Thompson as ""
 Jim Goodwin as "Farrell"

Uncredited co-stars 
 Nichelle Nichols as Uhura (voice only)
 Eddie Paskey as Leslie
 Unknown actors as
 Bobby
 
 Helmsman
 Communications officer
 Sciences technician 1
 Sciences technician 2
 Section chief (voice only)

Stunt double 
 Don Eitner as the body double for William Shatner

Stand-ins
William Blackburn as the stand-in for DeForest Kelley
Frank da Vinci as the stand-in for Leonard Nimoy
Jeannie Malone as the stand-in for Grace Lee Whitney
Eddie Paskey as the stand-in for William Shatner

References 
abort control circuit; Alfa 177; Alfa 177 canine; alter ego; animal; announcement; annoyance; answer; assignment; autopsy; ; base cycle; bed; body; body function panel; "Bones"; burnout; bypass circuit; cabin; casing; circuits; coadjutor engagement; coffee; communication line; compassion; confusion; consciousness; Constitution-class decks; courage; decontamination; ; dizziness; duty officer; Earth; engineering; engineering deck; evil; exposure; eye; face; fear; feeling; force of will (aka strength of will); frost; frostbite; full postmortem; geological technician; good; hand; hand phaser; hostility; hotline; Human; impostor; impulse engine; intellect; intelligence; kiss; landing party; leader; leader circuit; logic; love; lust; magnetism; main circuit; make-up; microtapes; mind; mirror; name; neck; opportunity; ore; overload; phaser weapon; plan; pot; protoplaser; power generator; quality; quarters (cabin); rape; rice wine; risk; rock; room; room service; rope; Saurian brandy; savage; search; search party; second officer; section; section chief; ship's manifest; shock; skiing season; specimen; specimen case; specimen gathering mission; status report; store; temperament; surface temperature; survival procedure; sympathy; synchronic meter; temperature; terror; theory; thermal heater; time sheet; training program; tranquilizer; transporter; transporter circuit; transporter malfunction; transporter technician; transporter room; transporter unit; transporter unit ionizer; truth; unconsciousness; vacation; velocity balance; violence; Vulcan nerve pinch; "watching the store"; week; wild man; ; yellow

Unreferenced materials 
; Heaven; Hell; malingerer; nuclear energy; nurse maid; power plant; rattle; traffic signal;

External links
 
 
 
 

de:Kirk : 2 = ?
es:The Enemy Within
fr:The Enemy Within (épisode)
it:Il duplicato (episodio)
ja:二人のカーク（エピソード）
nl:The Enemy Within
pl:The Enemy Within
ru:Внутренний враг (эпизод)
sv:The Enemy Within

TOS episodes