Kirk is beamed up from a planet, only to have second Kirk beam up a minute later. It is discovered Kirk has been split in half. The first Kirk is the intellect and gentleness, the second is unbridled lust, desire and aggression. The "bad" Kirk attacks crewman and attempts to rape Yeoman Rand. The "good" Kirk has trouble making important decisions as he is only half of himself. Meanwhile, Sulu and other crewmen are on the planet below-stranded because the Enterprise can't risk beaming them up until they fix the problem. Temperatures on the planet get to -120F and the men can't last long.
Finally, a solution is found and Kirk is back to his normal self. No dead "crewman number 6" in this one =).