This plot. Jimminy, this plot. A transporter accident [splits Kirk into a good but ineffectual twin and evil but charismatic twin], the crew has to [convince weak kirk and strong kirk to hug it out on a transporter pad] before [Sulu and an away team freeze to death]. A valuable lesson about [human nature] is learned. Replace the variables, and you've got three dozen other Trek episodes over the next fifty years. I shouldn't fault this one for being the first, but I can't help it.
Though we do get the cocker-spaniel-with-crap-stuck-to-it spacedog, and some very effectively creepy lighting and direction.