The Enemy Within
During a survey of planet Alfa 177, geological technician Fisher slips down a rock, gashing himself badly and slathering his uniform with an odd off-world magnetic type of yellow ore. Fisher beams up to the USS Enterprise for treatment after noticeably cutting his hand and bleeding from it. Detecting a curious overload in the transporter circuitry, Scotty has Fisher decontaminated before reporting to sickbay, but the problems have already begun; the strange yellow ore has altered the function of the transporter. Next, Captain Kirk beams up from the planet, before the fault is discovered. He apparently materializes normally and Scotty escorts the disoriented captain out of the room. Kirk is, in fact, a shadow of himself. Due to this transporter accident, Kirk has been split into two beings. The first that materialized embodies all of Kirk's positive qualities. Moments later, after everyone has left the transporter room, Kirk's evil twin materializes on the transporter pad and wreaks havoc on the crew. Roaming the ship, Kirk arrives at his Yeoman-Janice Rand's quarters quickly alone with Rand while drunk from Saurian brandy. The evil duplicate Kirk forcefully and heavily kisses her and pushes Rand to the floor attempting to rape her. But, she defends herself and leaves a large scratch on the evil duplicate Kirk's face.
Later in engineering, just as the evil Kirk is about to kill the good Kirk with a phaser, Spock knocks evil Kirk out with the Vulcan nerve pinch, but not before the phaser discharges and disables the transporter ionizer with one heavily concentrated phaser shot. This now makes it harder to rescue Sulu and the landing party who are trapped on the rapidly freezing planet Alfa 177.
Meanwhile, on the planet below, the remaining landing party is suffering through the increasingly bitter cold. Attempts to beam heaters and other support devices produce only non-functional duplicates. While in the Enterprise's briefing room, Kirk speaks on ships communication to Sulu, trying to reassure his helmsman, all the while he's becoming more indecisive of his own command abilities. Seeing that Kirk is struggling to help, Spock cuts in and tells Sulu to hold on for just a little while longer.
While in sickbay, evil Kirk is in pain vocalizing it with loud outbursts while being restrained on a bio-bed, in pain from his body functions having been weakened from the duplication process. Good Kirk takes his evil self's hand, tells him not to be afraid and to use his mind, rather than his savagery. Kirk ultimately survives split into two humans then returns to his original whole form, having his good and evil side enact. Later, both Kirk's are taken to the transporter room to be reintegrated as one being, returning to his original whole form.
The lesson of the story
The Enemy Within is a Star Trek episode which illustrates the idea of an intrinsic human dualism between good and evil. While realistic in its portrayal of what a man of fallen nature would do, its portrayal of ill-clad men trying to survive sub-zero temperatures is questionable, assuming the landing party survived with phasers to heat up rocks for warmth. Blankets could have been transported down, and the ship may have had a shuttle craft. The men made no attempt to build a shelter or even to shield their faces and hands from the cold. However survival and a physical form of the human condition endure as the Enterprise rescues Sulu and the others nearly frozen to death. Kirk ultimately survives split into two humans then returns to his original whole form, having his good and evil side enact.