The Enemy Within (Star Trek: The Original Series)

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"The Enemy Within"
Star Trek: The Original Series episode
STEnemy Within.jpg
The evil Kirk makes advances toward Yeoman Rand.
Episode no. Season 1
Episode 5
Directed by Leo Penn
Written by Richard Matheson
Featured music Sol Kaplan
Cinematography by Jerry Finnerman
Production code 5
Original air date October 6, 1966 (1966-10-06)
Guest actors
Episode chronology
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"The Naked Time"
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"Mudd's Women"
List of Star Trek: The Original Series episodes

"The Enemy Within" is an episode of the science fiction television series Star Trek. It was first broadcast on October 6, 1966. It is the fifth episode of the first season, written by Richard Matheson and directed by Leo Penn.

In the plot, a transporter malfunction causes the captain to be split into two individuals, a good and evil captain Kirk which come aboard separately, and stranding some of the crew of the Enterprise on the planet until a solution can be found.

The production could not afford to build a shuttle craft, which is why the transporter was so prominent. Transporter effects were inexpensive in comparison with shuttle effects. In this episode, Sulu and his landing party could easily have been saved by shuttle when the transporter was damaged. The shuttle craft would debut in a later episode entitled "The Galileo Seven"


On stardate 1672.1, the Federation starship USS Enterprise is on a geological exploration of the planet Alpha 177. Geological Technician Fisher falls from an embankment and injures his hand. He is immediately beamed back to the Enterprise for medical treatment. During the beamup, the transporter system behaves oddly. Nearly losing the technician, Chief Engineer Scott immediately checks over the transporter equipment, but finds nothing wrong. He only notices magnetic dust from some ore samples covering Fisher's uniform when the technician materializes. Scott orders him to have the uniform decontaminated.

Soon afterward, Captain Kirk beams back to the ship. The transporter seems to work smoothly, but Kirk feels disoriented. Scott escorts him out of the room, leaving it empty. A moment later, a second Captain Kirk materializes on the transporter pad and no one is aware of his arrival. This Kirk is the "other half" of the Captain's split persona: a physical manifestation of his more selfish and evil qualities.

The first thing the "evil" Kirk does is head to Sickbay, where he demands a bottle of Saurian brandy from Chief Medical Officer Dr. McCoy. McCoy doesn't understand this sudden, aggressive mood swing.

Back in the transporter room, Scott beams up an animal specimen from the landing party, which appears to be a small, horned, dog-like creature. Two "dogs", however, arrive on the transporter pads. One is extremely vicious, while the other is very docile, yet they appear physically identical. Confirming that the team only beamed one animal to the ship, Scotty realizes that something is very wrong with the transporter system. He is forced to strand the remaining landing party (including Lt. Sulu) on the planet until further notice.

Meanwhile, the evil Kirk, appearing drunk and disorderly, enters the quarters of Yeoman Janice Rand and lies in wait for her. When she arrives, he grabs and assaults her. She manages to fight back, scratching his face with her sharp fingernails, and then tries to escape. She cries out for Crewman Fisher to call Mr. Spock. The evil Kirk incapacitates Fisher before he can help. Simultaneously, elsewhere on the ship, the good Captain Kirk begins to show signs of weakness, apparently losing his ability to give orders and commands, the so-called "power of decision." With the help of Mr. Spock, the good Captain Kirk gives orders for all crewmen to arm themselves with phasers set for stunning force and locked and for all crewmen to be on the lookout for the impostor, the evil Kirk, who can be identified by the scratches on his face. The crewmen are not to injure the evil Kirk.

The evil Kirk, hearing the good Kirk's orders, hides the scratches on his face with makeup and acquires a phaser from Crewman Wilson, whom he also incapacitates, and then hides in the lower levels of the ship. Anticipating how his other half will behave, the good Kirk finds the evil Kirk on the Engineering Deck, and Spock disables the latter with a Vulcan nerve pinch. Spock observes the evil Kirk showing signs of fatigue. It is quickly surmised that neither Kirk can survive for long in his separated state. Time is running out not only for the Kirks, but also for the stranded landing party, the members of which are slowly freezing to death as night falls on Alpha 177.

Scott reports that the transporter unit ionizer is damaged and would normally take a week to repair; however, he and Spock rig up a connection to power the transporter from the ship's impulse drive. They recombine the dog-creature, but it dies as a result of the strain. Not giving up hope, Scotty continues to work on the problem.

In the meantime, the good Kirk releases his opposite's bindings in Sickbay when the evil Kirk promises not to fight. He promptly overpowers the good Kirk and rushes off to the bridge, where he orders the ship to leave orbit convincing the navigator that the landing party can't be saved. The two Kirks confront one another on the bridge, and "evil" Kirk collapses whereupon "good" Kirk rushes him to the transporter room. With fingers crossed, Spock dematerializes both Kirks, and finally a single Kirk returns. Demonstrating that his power of command has returned, along with his intelligence and compassion, Kirk's first words are: "Get those men aboard fast." The landing party members are beamed up, and, aside from a "little" exposure and frostbite, they are fine.

Back on the bridge, Kirk is himself once again. He tells Spock: "Thank you, Mr. Spock — from both of us." When asked what to tell the crew, Kirk says that the intruder is back where he belongs and to leave it at that.


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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