Star Trek (1966–1969)
24 user 6 critic

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)

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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 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 »






Release Date:

6 October 1966 (USA) See more »

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Did You Know?


In this episode we get to follow Kirk behind the large engine room machinery components in the first trip to the engineering deck (which dialog identifies as being in the lowest parts of the ship). To allow this to happen, the new set had to be temporarily expanded to hide the sound stage beyond it. After the double is rendered unconscious by the first neck pinch in the series, the quickly-assembled wall behind the three characters can be observed to have a very rough edge where it meets the floor. Pieces of sets that were designed to be added and subtracted easily were called "wild." Although Kirk pursues Ben Finney into these components in Star Trek: Court Martial (1967), this is the only time we get to see the space behind them. The view of the tubed structures behind the grille was a forced perspective set. The tubed machinery appears to be many dozens of meters long, but this is an illusion created by making each vertical piece much smaller than the one in front of it. Diminishing numbers were later printed on the tubes immediately behind the grid to add to the illusion. In episodes where the engines were under stress, lighting effects were used inside the tubed-machinery room. The set was extensively remodelled between the first and second seasons. See more »


When Sulu, stuck on Alpha 177, comments on his communicator about "the skiing season opening down here" to Good Kirk in the briefing room, Spock enters, sits at the computer console and interrupts their discussion, pressing his communicator button on his console. The communication console's white "in use" light does not illuminate yet Spock clearly makes contact with Sulu. See more »


Sulu: Temperature's starting to drop?
Captain James T. Kirk: Yeah. At night it gets down to 120 degrees below zero.
Sulu: That's nippy.
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Referenced in Mind Meld: Secrets Behind the Voyage of a Lifetime (2001) See more »


Theme From Star Trek
Written by Alexander Courage
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User Reviews

Kirk and Hyde
30 May 2018 | by Gerardrobertson61See all my reviews

Interesting story line which sees a good and evil Kirk on the Enterprise. What I found most disturbing was the attempted rape of Yeoman Rand and how it was just blow off, (pardon the pun) as if woman is the 60's or in this case, 23rd Century just accept it. Spocks comments at the end of the episode was sexist and defiantly in bad taste.

The whole premise of Jekyll and Hyde is shown in this episode, although there is not the murder of rape of women, which would have taken this episode down a darker path.

Not one of the best of Season 1 but an interesting one.

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