A transporter malfunction splits Captain Kirk into two halves: one meek and indecisive, the other violent and ill tempered. The remaining crew members stranded on the planet cannot be beamed up to the ship until a problem is fixed.
While beaming back aboard the Enterprise, a transporter malfunction results in two vastly different Captain Kirks being beamed aboard. His personality has in effect been split into two. One Captain Kirk is weak and indecisive, fearful of making any kind of decision; the other is a mean-spirited and violent man who likes to swill brandy and force himself on female crew members. Meanwhile, as Scotty struggles to repair the transporter, the landing party is stuck on the planet below with temperatures falling rapidly.
- With his landing party stranded on a lethally cold planet, Kirk is 'split' into two Kirks, one evil, and one good.
The good Kirk, weak because he lacks half of his persona, does not know that his evil Twin is prowling the ship, seducing his pretty Yeoman, who resists and scratches him, beating up a crewman who is a witness to his near-raping of the Yeoman.
Hearing the evidence, Spock knows there is an evil Twin of Kirk roaming the ship. They could go back to a Starbase and make repairs, but then the men below, including Sulu, would perish in the cold at night. The only solution is to somehow merge these two beings.
First, after repairs are made, they try the experiment with a unicorn-dog, which was split into two when it was beamed up. One half of the unicorn-dog was peaceful, the other was snarling and biting.
After trying the experimental merge the dog returns intact as one dog, but the shock has killed it. "Suppose this happens to you!!" McCoy yells. Kirk is lost, he does not know what to do. They all understand why.
Although it is important to have goodness for intellect and mercy, it is the evil side that provides decisive action and strength. One, without the other, cannnot survive.
Meanwhile, the evil Kirk, who escaped after being caught, takes over the bridge, and gives insane commands. Kirk and Spock confront him. Kirk subdues him, and they take him to the transporter and try the 'merge.'
Kirk reappears in one body, dizzy. Spock is worried. "Captain?" he asks gently. Kirk snaps his face up "Yes, it's me, Spock. Now let's beam those men the hell up home."