1.5 The Enemy Within

Overview
Captain Kirk is split into aggressive and timid copies in a transporter mishap.

Plot Summary
Stardate: 1672.1-1673.1 First appearance of the space dogOn the planet of Alfa 177, the crew of the Enterprise scan the planet. When Geological Technician Fisher is injured, he is allowed to beam up to the ship for medical aid. A malfunction in the ship's transporter occurs when it beams up Fisher, who is covered in a magnetic ore. Scotty checks the transporters, but does not find anything wrong with it. He beams Captain Kirk aboard and there is another malfunction as Kirk is suspended in animation at the transporter. Soon, Kirk is able to move again and everyone in the room leaves. Then another Kirk appears at the transporter and gives an menacing glance at his surroundings.

When the crew decides to beam up the dog, they discover that the transporter had separated the dog into two opposites and conclude that the same can happen to a man. Meanwhile the Evil Kirk attempts to sexually assault Janice. During the assault, she manages to scratch her attacker. The assault is interrupted by Fisher who ran away to alert Spock. As he alerts Spock, the Evil Kirk beats him with his fists and hides himself. Janice is questionedWhen Spock does arrive, he takes the good Kirk with him and questions Janice as to what had happened. Janice describes what had happened to her and mentions that she had scratched the Evil Kirk. Thew crew notice that the Good Kirk does not have any scratches on his face and they become further confused when Fisher comes into the room to accuse Kirk of assaulting Janice. Spock dismisses the crew members to their duties and concludes that an impostor is aboard the ship.

Without the transporter in order, the the Enterprise is force to leave its crew members on the planet until it can repair the transporter. With the planet's freezing temperatures at night, Captain Kirk and the Enterprise was a time limit to catch the duplicate and fix the transporter. The Good Captain Kirk orders a search party to find the Evil Kirk. He instructs the crew to identify the Evil Kirk by looking for the scratches on his cheek and to keep their hand phasers on stunning force. When the Evil Kirk sees the Good Kirk's message, he becomes enraged and covers the scratches with Janice's make-up. He then steals a hand phaser from one of the crew members and leaves the room. Evil Kirk winces in painSpock and the Good Kirk decide to head to the lower levels of the ship to find the Evil Kirk because that is what the Good Kirk would do in his situation. When the reach their destination, The Good Kirk and Spock split up to cover more ground. The Good Kirk is able to find the Evil Kirk and he is able to corner him while Spock sneaks up from behind to knock him out with the Vulcan Nerve Pinch.

When the Evil Kirk is put in the sick bay, McCoy discovers that the Evil Kirk is dying due to the transporter weakening his body. Meanwhile, the Good Kirk's ability to lead weakens and he becomes convinced that that he must become one with the Evil Kirk again. McCoy assures Kirk that every man had a dark half and needs it to function.

Scotty soon adjusts the transporter and tests it by placing the dog on the transporter. The dog dies and Kirk is faced with a tough choice. He can either risk dying on the transporter or order the remains of the dog to be examined to see what had gone wrong while his men on the planet brave the cold temperatures of the planet's nighttime. Ultimately, Kirk chooses to go through the transporter and try to unify himself. When the Good Kirk releases the Evil Kirk from his bondages, the Evil Kirk steals the Good Kirk's phasers and heads to the control deck. Soon the Good Kirk also arrives in the control deck and the crew members are confused. Spock orders the crew to stand down while the captains sort out their mess. The Good Kirk is able to subdue the Evil Kirk and brings him to the transporter so they can become one once more. Kirk is back in commandThe transporter is turned on and the two Kirks are reunited as one being. When Captain Kirk steps out of the transporter, he quickly orders that the men on the planet should be teleported to the Enterprise posthaste. The men trapped on the planet are teleported successfully and have suffered no major injury. Kirk then orders the helmsman to resume their course.

Production Notes
Originally, the script for this episode did not contain a subplot where Sulu and three other crew members are trapped on the freezing planet. This plot was added by staff writers and was generally disliked by Richard Matheson, who had written the script.

To help the audience differentiate between the two Kirks, eyeliner was given to the Evil Kirk to ephasize the starkness of his gaze. William Shatner also wore a wear green shirt to let the audience know when he was playing the Good Kirk.

It is also worth noting that the Enterprise did not attempt to land on the planet to rescue the crew members even though it was would have made the situation less complicated. This could be attributed to how the writers were unsure at this point what the capabilities of the Enterprise were or if the writers did not have the budget to attempt such a scene.