"The Enemy Within" is an episode of the American science fiction television series Star Trek. It was first broadcast on October 6, 1966. It is the fifth episode of the first season, written by Richard Matheson and directed by Leo Penn.

On stardate 1672.1, 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 the 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 apparently works correctly, but Kirk experiences some disorientation, and Scott escorts Kirk out of the transporter room. They are unaware when 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 but assume he is the real captain.

Scott assists in beaming a dog-like animal specimen from the planet, but he and the others are surprised when two identical creatures materialize - 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 a good and evil counterpair. Scott reports this to Mr Spock and then orders the transporters 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, recognizable due to the scratches on his cheek, but not to harm him.

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