The Enemy Within (Star Trek: The Original Series)

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"The Enemy Within"
Star Trek: The Original Series episode
Episode no.Season 1
Episode 5
Directed byLeo Penn
Written byRichard Matheson
Featured musicSol Kaplan
Cinematography byJerry Finnerman
Production code5
Original air dateOctober 6, 1966 (1966-10-06)
Guest appearance(s)
  • Jim Goodwin - Farrell
  • Edward Madden - Fisher
  • Eddie Paskey - Connors
  • Garland Thompson - Wilson
  • Don Eitner - photo double for Capt. James Kirk
Episode chronology
← Previous
"The Naked Time"
Next →
"Mudd's Women"
Star Trek: The Original Series (season 1)
List of Star Trek: The Original Series episodes

"The Enemy Within" is the fifth episode of the first season of the American science fiction television series, Star Trek. Written by Richard Matheson and directed by Leo Penn, it first aired on October 6, 1966.

A transporter malfunction causes Captain Kirk to be split into two people, one "good," but weak and ineffectual; the other "evil," but strong and decisive.


On stardate 1672.1, the Federation starship USS Enterprise is on a geological exploration of the planet Alpha 177. Geological Technician Fisher is injured after a fall and transported aboard the Enterprise, though Chief Engineer Scott has some trouble with the transporter. The transporter equipment appears to be fine but he notices some magnetic dust from ore samples covering Fisher's uniform that may have interfered with the transport and the uniform is decontaminated.

Later, Captain Kirk transports back to the ship. The transporter appears to work correctly, but Kirk experiences some disorientation, and Scott escorts Kirk out of the transporter room. They are unaware when the transporter activates a second time, materializing a second version of Kirk which behaves more maliciously than his counterpart. This "evil" Kirk begins to wander the ship, and those he encounters are confused by his behavior.

Scott assists in beaming a dog-like animal specimen from the planet, but he and the others are surprised when two identical creatures materialize – one completely docile and the other vicious. Scott surmises that the ore dust has caused the transporter to split the personalities of those they beamed up, creating good and evil counterparts. Scott reports this to Mr Spock and then orders the transporter taken out of service to investigate, stranding the landing party on the planet as the bitterly cold night sets in. Elsewhere on the ship, the "good" Kirk begins to feel uncertain, and is unable to make decisions that could affect the safety of the ship and crew. Meanwhile, the "evil" Kirk, in a drunken state, attempts to assault Yeoman Janice Rand in her quarters, but she manages to scratch his face with her fingernails. When she tries to escape and gain help from Fisher, the "evil" Kirk attacks and knocks him out. Rand is able to report the incident to the bridge, and the "good" Kirk, with Spock's help, orders security to try to capture the "evil" Kirk; the crew are told of an imposter recognizable by the scratches on his cheek, but not to harm him.

The "evil" Kirk hears this announcement and finds makeup to mask his injury, while securing a phaser from another security officer, before going into hiding in Engineering. The "good" Kirk considers what his evil self would do, and correctly locates the other Kirk. Though there is a brief scuffle, Spock is able to disable the "evil" Kirk with a Vulcan nerve pinch. Spock recognizes both Kirks are suffering fatigue, and they must find a way to reverse the transporter accident, not only to save Kirk but also the landing party stranded on the planet. Spock and Scott attempt to use power from the ship's impulse drive to reverse the transporter on the dog-like specimen, and while the two beings are rejoined as one, the creature dies from the stress. The two continue to work on making sure Kirk will not suffer the same fate.

In Sickbay, where both Kirks wait, the "evil" Kirk appeals to the "good" Kirk's compassion, to release his bonds, allowing him to overpower his other self and escape to the bridge. There, he orders the crew to leave orbit, telling the navigator that the landing party cannot be saved. The "good" Kirk and McCoy race to the bridge (before the "evil" Kirk's order is carried out), where the two Kirks face off. The "evil" Kirk falters from weakness and the "good" Kirk uses the opportunity to rush him to the transporter room and orders Scott to attempt the reversal process again despite not having tested their improvements fully. Distressed, the "evil" Kirk begs for mercy, claiming he only wants to live, to which the "good" Kirk reassuringly replies they will both live on as parts of each other. The process is completed successfully, with Kirk rejoined as one being. With his sense of command and good will restored (and the transporter repaired), Kirk quickly orders the landing party beamed up, where they are found to be safe despite the cold. Kirk gives Spock his thanks – "from both of us".


This episode is one of the small group for which a full score was written, in this case by Sol Kaplan. Jeff Bond notes:


Zack Handlen of The A.V. Club gave the episode an 'A-' rating, noting that while the last act of the episode was somewhat redundant, the first two run smoothly, and describing Shatner's acting as Kirk's good half as "very solid stuff."[2]

In 2014, IO9 rated "The Enemy Within " the 81st greatest episode of Star Trek.[3] In 2016, IGN ranked "The Enemy Within" the 7th best episode of the original series.[4]

In 2015, WhatCulture ranked this the 21st best episode of all time in the Star Trek science fiction universe.[5]

In 2016, Empire ranked this the 15th best out of the top 50 episodes of all the 700 plus Star Trek television episodes.[6]

In 2016, Radio Times ranked the moment where Kirk exclaims "I'm Captain Kirk!!", the tenth best moment of all Star Trek film and television up to that time.[7] They note that in this episode Captain Kirk is split into two people, with Shatner playing both parts.[7] They felt that Shatner's acting performance "completely carries this early first season episode."[7] They note that the evil version attacks Yeoman Rand and pretends to be the good Captain.[7]

In 2017, Fatherly ranked this episode as one of the top 10 episodes for kids to watch.[8]

In 2018, PopMatters ranked this the 8th best episode of the original series.[9] They praised Shatner (who plays both Kirk, and his sinister transporter clone) for his nuanced yet over-the-top performance, depicting an extraordinary range of emotions.[10]

A 2018 Star Trek binge-watching guide by Den of Geek, recommended this episode as one of the best of the original series.[11]

In 2019, Nerdist included this episode on their "Best of Kirk" binge-watching guide.[12]


  1. ^ Bond, Jeff (1999). The Music of Star Trek: Profiles in Style. Lone Eagle. ISBN 1580650120.
  2. ^ Handlen, Zack (January 22, 2009). ""The Enemy Within"/"Mudd's Women"". The A.V. Club. Retrieved June 12, 2009.
  3. ^ [1]
  4. ^ Staff, I. G. N. (September 5, 2016). "The Top 10 Classic Star Trek Episodes". IGN. Retrieved March 27, 2019.
  5. ^ Fisher, Matthew (April 28, 2015). "30 Best Star Trek Episodes Of All Time". Retrieved July 15, 2019.
  6. ^ "The 50 best Star Trek episodes ever". Empire. July 27, 2016. Retrieved June 29, 2019.
  7. ^ a b c d "The 50 Greatest Star Trek moments of all time - 8". Radio Times. Retrieved July 24, 2019.
  8. ^ "The 10 Best 'Star Trek' Episodes to Watch With Your Kids". Fatherly. October 31, 2017. Retrieved July 4, 2019.
  9. ^ "The 20 Best Episodes of 'Star Trek: The Original Series'". PopMatters. July 16, 2018. Retrieved July 8, 2019.
  10. ^ "The 20 Best Episodes of 'Star Trek: The Original Series'". PopMatters. July 16, 2018. Retrieved July 8, 2019.
  11. ^ "Star Trek: An Episode Roadmap for Beginners". Den of Geek. Retrieved July 24, 2019.
  12. ^ "A Guide to Binge Watching 7 Great STAR TREK Arcs". Nerdist. Retrieved July 15, 2019.

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