Star Trek: Season 1, Episode 5: “The Enemy Within”

 Summary:

Due to a yellow magnetic ore affecting the transporter, Capt Kirk has split in two--an unobserved, evil alter-ego appears aboard, demanding and drinking saurian brandy.  He assaults various crew members.  Meanwhile, Sulu and other crew are freezing below.  The positive Kirk loses his will to command.  The negative loses the social graces.

Analysis with spoilers:

While it makes for better drama, the crew could/should have died on the surface. Rather, someone should have beamed up the crew and stunned the violent ones who appeared as they appeared in the transporter. Then they could have repaired the transporter without putting lives in danger. But it would have lacked drama if they'd done the logical thing.

They use the transporter to meld the two parts back together.

What’s fascinating here is two-fold:

1. The question of who is the imposter (is even the whole self the imposter?)
2. A person needs the negative and positive aspects of the self to be fully functional, suggesting that some may have an overabundance of one aspect over another, but that such negative aspects serve a function. See also Note #2.

Author: 

Richard Matheson

Notes:

1. Alien is clearly a dog in a costume--a unicorn/lion-fur suit with antennae. The crew carrying around the unicorn dog becomes a visual running gag. Possibly it is not intentional. Simliarly, though, a spider dog frightened half a dozen hapless victims. So convincing costumes must be relative (although this one from the front might have frightened me, too):
2. This theme compares with the Khan episode, Star Trek Season 1, Episode #22, where the crew dislikes the man yet admires the negatives of Khan, a former tyrant.