"The Enemy Within" is the fifth episode of the first season of the American science fiction television series, Star Trek. Written by Richard Matheson and directed by Leo Penn, it first aired on October 6, 1966. 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 indecisive and ineffectual; the other "evil," impulsive and irrational. Plot The 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. 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. While unsupervised, the transporter activates a second time, materializing 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 behavior. Scott assists in beaming a dog-like animal specimen from the planet, but two identical creatures materialize (the "good" one then the "evil" one, like Kirk) – 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 struggles to make decisions. The "evil" Kirk, in a drunken state, assaults Yeoman Janice Rand in her quarters. She scratches his face with her fingernails. When Fisher witnesses this and calls security, the "evil" Kirk attacks and knocks him out. Rand reports the incident to the bridge. The "good" Kirk orders the crew to capture the "evil" Kirk, but at Spock's advice he keeps the fact that their quarry is his evil half a secret so as not to weaken the crew's faith in him; the crew are instead told of an imposter recognizable by the scratches on his cheek. The "evil" Kirk