Trama

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.—garykmcd

While beaming up from planet Alpha 177 (Enterprise had visited there for a specimen gathering mission), a transporter accident splits Captain Kirk into two beings: one "good", who is weak and indecisive, and one "evil", who is overly aggressive and domineering. This is not noticed at the time as the evil Kirk appeared a few seconds later and Scotty was escorting the good Kirk out of the room at the time. Scotty finds out about the malfunction when one of the animals beamed up after Kirk split into 2, who are not duplicates, but opposites of each other

Good Kirk puts the pieces together (with reports of Kirk assaulting McCoy for Saurion Brandy and so on) and deduces that he himself has split into 2. Evil Kirk tries to rape Janice (Kirk has always had a thing for Janice, but had never acted on his impulses before due to the constraint from his good self, which was now removed) and hits a yeoman unconscious, when he sees this and then runs away to try and warn Spock. Janice scratches evil Kirk's face. Spock deduces that there is only one logical answer that there is an impostor on-board. Spock says that they cant kill the evil Kirk as they don't know the impact it could have on the good Kirk Meanwhile 4 men from the ship are stuck on the surface of the planet and cant be beamed up since Scotty has not figured out how to fix the transporter yet. Temperatures are dropping and it gets to minus 120 degrees at night. They will die

Meanwhile Good Kirk makes an announcement to the crew over the PA about the impostor. The evil Kirk listens to the broadcast and covers his scratch. Spock understands that the evil Kirk is still Kirk and asks the good Kirk where he would hide in the ship. They both head to the lower engineering levels where they find the evil Kirk hiding. As Evil Kirk starts to shoot the good Kirk, Spock captures him with the Vulcan neck grip. Before falling unconscious, the evil Kirk fires his phaser which hits the transporter circuit boards. Spock observes that the good Kirk is quickly losing his ability to make decisions and says that its the evil Kirk, that makes Kirk a great leader.

Meanwhile the transporter will require a week to get fixed and men below are dying by every passing hour. good Kirk is indecisive.. Meanwhile the evil Kirk is dying in the sick bay as its internal organs were injured during the duplication process. Good Kirk talks to his evil self and gives it reason to live. Scotty figures out a way to make the transporters work by using the impulse engines. They send the duplicate animals through the transporters and then reverse it and find that it comes back as a single self. The animals dies. Spock says its the shock of putting the animal together that killed it, while McCoy wants to do a full autopsy on the dead animal before proceeding any further. Spock wants Kirk to go through the transporter, but McCoy is afraid it might kill him.

good Kirk wants help in making the decision, but is not yet willing to relinquish command. Evil Kirk escapes the sick bay (by hitting the Good Kirk and stealing his uniform) and takes control of the bridge and orders the Enterprise out of orbit, abandoning the team on the planet. The good Kirk reaches in time and realize that they are both halves of each other and cant survive in isolation. They head to the transporter room. Kirk comes back as one. He orders the rescue of his team on the surface.