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 evil and one good.


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 apparently works 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 a good and evil counterpair. Scott reports this to Mr Spock and then orders the transporters 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, recognizable due to 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, "Although he wrote only two scores for the series, New York composer Sol Kaplan's music was tracked endlessly throughout the show's first two seasons. ... 'The Enemy Within' is a thrillingly intellectual score, by turns overcome with compassion for Kirk's plight and clinically detached in its melodic experimentation with the situation... [T]he aggressive, threatening 'evil Kirk' music made major contributions by being tracked into other episodes."[1]


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]


  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.

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