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The Enemy Within 

A transporter malfunction splits Captain Kirk into two halves: one meek and indecisive, the other violent and ill tempered. The remaining crew members stranded on the planet cannot be beamed up to the ship until a problem is fixed.


Leo Penn


Richard Matheson, Gene Roddenberry (created by)


Episode complete credited cast:
William Shatner ... Capt. Kirk
Leonard Nimoy ... Mr. Spock
DeForest Kelley ... Dr. McCoy
Grace Lee Whitney ... Yeoman Janice Rand
George Takei ... Sulu
James Doohan ... Scott
Ed Madden Ed Madden ... Fisher (as Edward Madden)
Garland Thompson Garland Thompson ... Wilson
Jim Goodwin ... Farrell




While beaming back aboard the Enterprise, a transporter malfunction results in two vastly different Captain Kirks being beamed aboard. His personality has in effect been split into two. One Captain Kirk is weak and indecisive, fearful of making any kind of decision; the other is a mean-spirited and violent man who likes to swill brandy and force himself on female crew members. Meanwhile, as Scotty struggles to repair the transporter, the landing party is stuck on the planet below with temperatures falling rapidly. Written by garykmcd

Plot Summary | Plot Synopsis


TV-PG | See all certifications »

Did You Know?


Kirk's alternate green wrap-around uniform was introduced so that the audience would be able to tell the difference between the good Kirk and the evil Kirk. See more »


When "good" Kirk supports his double on the transporter pad, his left hand is lower than his right. It then cuts to a very brief shot of Spock and then back to the Kirks, and the left hand is now higher than the right. See more »


Lt. Cmdr. Leonard 'Bones' McCoy, M.D.: Jim, you can't risk your life on a theory!
Mr. Spock: 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 out over both, makes them live together.
[to Kirk]
Mr. Spock: Your intelligence would enable you to survive, as well.
See more »

Alternate Versions

Special Enhanced version Digitally Remastered with new exterior shots and remade opening theme song See more »


Featured in For the Love of Spock (2016) See more »


Theme From Star Trek
Written by Alexander Courage
See more »

User Reviews

Psychological "thriller"
29 May 2009 | by BlueghostSee all my reviews

This episode explores the "duality" of man. The ying-yang of how and what drives and keeps a man sane. Shatner is challenged as an actor to give us two extremes of one character; a bipolar portrayal, so to speak. The audience witnesses the theory of what drives a man, and that which stables his more aggressive tendencies.

Is the premise true? A single Star Trek episode certainly cannot answer nor adequately extrapolate in a single dramatization, but it is an interesting character study from a purely psychological point of view. While other TV shows were concerned about what dad would do when he got home, Star Trek was examining deep human issues on all levels emotional and scientific levels (or as could be expected by a dramatization of science fiction).

Spock sums up the episode's and author's thrust near the end. And Kirk makes the final observation after being thrust into a kind of psychological rehabilitation courtesy Scotty's "finicky piece of machinery", remarking on a man's self observance. The idea here is to view ones' various personality traits, reign them in, then expunge them to reconfigure the subject back to psychological norms.

Not an episode that comments on any real deep social issues, but one that asks the viewer to look at a fractured man and his reconstitution.

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Release Date:

6 October 1966 (USA) See more »

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