Amazon.com
Written by renowned novelist-screenwriter Richard Matheson (
The Incredible Shrinking Man), the outstanding episode "The Enemy Within" proposes a transporter malfunction that results in Captain Kirk being divided into two versions of himself, one aggressive and brutal, the other sensitive and good. Essentially, the personality mix that makes Kirk an effective leader and balanced man is scattered like so many marbles, and the result is one captain running around mauling women and wreaking havoc while the other is frightened and indecisive. The production is very effectively done, and William Shatner's performance is among his most interesting.
--Tom Keogh
From the Back Cover
A transporter malfunction causes both Kirk and a dog-like creature to split into separate beings: one compassionate, the other savage.
TREK TRIVIA
Leonard Nimoy devised the FVNP (Famous Vulcan Nerve Pinch) for this episode, believing that Spock would not hit the captain with the butt of his phaser.
Note that when the two Kirks beam in, neither is wearing an insignia, but when we see them later, they are! Watch for Sulu using his phaser to heat the rocks... it's the only time we see a phaser firing multiple beams.