Star Trek (1966): S01E05 – The Enemy Within


“The Enemy Within” is the fifth episode of the first season of the American science fiction television series, Star Trek.

In the episode, while beaming up from planet Alpha 177 a transporter malfunction causes Captain Kirk to be split into two people, one “good,” but weak and ineffectual; the other “evil,” but strong and decisive.

This episode prompted Leonard Nimoy to invent the Vulcan nerve pinch. It was the first episode in which Dr. McCoy says, “He’s dead, Jim”, and the first occasion on screen where Kirk was duplicated in some fashion.


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 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 appears to work correctly, but Kirk experiences some disorientation, and Scott escorts Kirk out of the transporter room. They are unaware that the transporter activates a second time, materialising 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 behaviour.

Scott assists in beaming a dog-like animal specimen from the planet, but he and the others are surprised when two identical creatures materialise – 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 good and evil counterparts. Scott reports this to Mr Spock and then orders the transporter 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; the crew are told of an impostor recognisable by 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 recognises 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”.

Star Trek TV Series

You can find a full index of Star Trek TV series here.

Star Trek TV Series, Films, and Documentaries

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Production & Filming Details

  • Director(s): Leo Penn.
  • Production: Desilu Productions (1966-1967) and Paramount Television (1968-1969).
  • Distributor(s): Paramount Pictures (1966-2006), CBS Paramount Television (2006-2007), and CBS Television Distribution (2007-Present).
  • Original Network: NBC.
  • Release Date: 06 October 1966.
  • Running Time: 50 minutes.
  • Country: US.
  • Language: English.

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