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The Enemy Within
Star Trek Original series, season 1, episode 5

Originally aired Oct. 6, 1966
Writer Richard Matheson
Director Leo Penn
Guest cast Jim Goodwin (John Farrell)
Edward Madden (Fisher)
Garland Thompson (Wilson)
The evil Kirk seizes Yeoman Rand
Sulu and crewmen huddle for warmth
Sulu and crewmen huddle for warmth
Kirk supports his dark opposite
Kirk supports his dark opposite
Stardate: 1672.1

Plot summary

Kirk, Sulu, and other crew members are gathering specimens on planet Alpha 177, where the night-time temperature dips as low as minus 120 degrees. Geological technician Fisher beams up following a minor accident. An unusual magnetic yellow ore, some of which is adhering to his uniform causes a problem with the transporter, which appears to resolve itself. Kirk (1) beams up next and arrives on the transporter pad aboard the Enterprise feeling dizzy. Scott escorts him away. Meanwhile a wild-looking, duplicate Kirk (2) materializes.

Kirk 2 goes to sick bay and demands saurian brandy. With the bottle in hand he then heads for Yeoman Janice Rand's quarters. Spock visits Kirk 1 following McCoy's report of the captain's strange behavior. Scott then tells Kirk that a dog-like specimen animal, beamed up from the surface, arrived as two individuals, one placid, the other wild and ferocious. "If this should happen to a man," muses Kirk.

Kirk 2 forces his attentions on Yeoman Rand who scratches his face. As Kirk leaves her quarters he is seen by Fisher who tries to alert the bridge but Kirk 2 knocks him unconscious. Rand reports what happened. Spock concludes: "There can be only one answer. We have an imposter aboard." Kirk 1 begins to investigate. Meanwhile, because of the transporter failure, Sulu and the rest of the landing party are stranded on the planet's surface where the temperature is plummeting.

Kirk 1's powers of decision-making are starting to falter. He asks Spock to correct him if he makes mistakes. Kirk 1 announces to the crew that an imposter is aboard and has scratches on his phase. Kirk 2 overhears and goes wild, screaming "I'm Captain Kirk!" He masks the scratches with makeup and takes a phaser from a crewman.

Kirk 1 tells Spock that if he were Kirk 2, which in some sense he is, he would headed for the lower levels of the ship – Engineering. There the two Kirks meet. Kirk 1 says: "You can't hurt me ... I'm part of you, and you of me." Spock comes upon Kirk 2 from behind and gives him the Vulcan neck pinch. With Kirk 2 restrained in the sick bay, Spock points out the opportunity that exists to study the roles of good and evil in human nature. An exceptional leader, such as Kirk, he concludes, depends on his negative side. Properly controlled and disciplined, he explains, it is crucial to the power of command.

With the transporter out of action, the good Kirk's willpower failing, and the condition of the landing party growing deteriorating, the situation looks grim. Kirk 2 also seems close to death, his bodily processes weakened by the duplication. Kirk 1 reassures him and urges him to use mind to hold on.

Spock reports that a way may have been found to fix the transporter. The two benign and savage versions of the specimen animal are put through the transporter as a test but although the creature comes back as one it is dead. Spock believes the animal died of shock because, lacking an intellect, it experienced blind terror. The captain, he explains, could control that fear. He likens the split individuality of Kirk to his own human and vulcan halves which are able to coexist because of the control of mind. McCoy counters by pointing out the risks of using the transporter. Kirk wavers but faced with the prospect of relinquishing command, and with the condition of the men on the surface, he comes to a decision and orders the transporter room to be made ready.

Kirk 1 releases Kirk 2, but Kirk 2 attacks him and knocks him out. Kirk 2 goes to the bridge and orders the Enterprise to be taken out of orbit. Spock questions his command. Kirk 1 and McCoy arrive on the bridge and Kirk 2's identity quickly becomes apparent as he flies into a rage. Kirk 1 takes his dark counterpart's weapon without resistance. "Can half a man live?" he asks. Kirk 2 pleads "I want to live." Kirk 1 replies "Yo will – both of us." He helps his alter-ego to the transporter room and the two Kirks are successfully reintegrated. "I've seen a part of myself no one should ever see," he remarks.

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