The story of the second Star Trek pilot, Where No Man Has Gone Before, is a familiar one: Man acquires god-like powers, man goes bat-shit crazy, man does nasty things with previously mentioned god-like powers, absolute power corrupts absolutely, blah, blah, blah. In this particular retelling of this story, the Enterprise attempts to leave the galaxy and runs into an energy barrier which kills nine crew members and knocks unconscious Enterprise helmsman and Kirk's best friend, Gary Mitchell, and ship's psychiatrist, Dr. Elizabeth Dehner. Mitchell begins to manifest incredible powers and it becomes apparent that these powers are going to his head. Science officer Spock advises Kirk to kill Mitchell before he gets too powerful. If things weren't bad enough, it's obvious from the beginning that Mitchell's grasp of reality before he got his god-like powers was shaky to begin with as he refers to Dr.Dehner as a "walking freezer unit" shortly after his character is introduced; as played by Sally Kellerman, Dr. Dehner oozes an almost indecent amount of sex appeal in every scene in which she appears. Clearly there was something seriously wrong with Mitchell before he acquired his powers. Or else he was socializing with some pretty interesting women before the Enterprise tried to leave the galaxy.
Dr. Elizabeth Dehner aka the “walking freezer unit”
Being a pilot, there are some aspects of Where No Man Has Gone Before which are different from the rest of the series. There is no Dr. McCoy, although there is a Dr. Piper, who is an old codger of a ship's surgeon. There is no LieutenantUhura. Sulu is not the swashbuckling helmsman he was in the rest of the series but is the ship's rather earnest physicist. Spock is almost emotional in the pilot, smirking rather smugly at Kirk when he feels he's got him boxed in while they're playing chess and looking downright irritated when Kirk not only escapes from his predicament but wins the game. Kirk isn't the decisive starship captain he was in the series but displays almost Hamlet-esque indecisiveness in Where No Man Has Gone Before; when Spock initially advises him to kill Mitchell, Kirk doesn't react quite as strongly as you'd think someone would react after being told that the logical thing to do is to kill his best friend, probably because he agrees with Spock but just can't bring himself to translate thought into action; this is supported by his insistence on going after Mitchell alone at the climax of Where No Man Has Gone Before and his stated belief that it's his own fault that things have deteriorated as far as they have. However, perhaps this is nitpicking; even the most decisive person in the world is going to hesitate when faced with the realization that he has to kill his best friend. At least I hope that would be the case.
There are other differences besides those in characters and characterization. The color scheme of the uniforms is not yellow, red, blue but yellow, tan, blue and the material used is different, thicker, making the cast look like they're wearing sweaters. And there is no Vulcan nerve pinch. Or at least that's the impression I got since Kirk and Spock, when faced with the problem of how to subdue a drunk-with-power Gary Mitchell, settle for the rather unsophisticated method of beating him senseless with good old-fashioned punches after which they tackle him and pin him down so that Dr. Dehner can sedate him. After the Vulcan nervepinch was introduced in the series, a scene like this would've played out with Kirk somehow distracting Mitchell, allowing Spock to pinch him into oblivion.
Despite these differences, Where No ManHas Gone Before is recognizably Star Trek with its somewhat thoughtful story of the corrupting influence of power but it doesn't quite work without the now well established dynamic of McCoy and Spock debating the emotional and logical sides of the problem at hand with Kirk mediating between the two. One could argue that Dr. Dehner takes the role that was eventually McCoy's in the series as she passionately advocates for Mitchell as the rest of the crew, even his best friend Kirk who owes him his life, grows more and more wary of him. But it doesn't quite click. I think it has something to do with Spock's characterization in the second pilot. As I mentioned before, he's almost emotional in Where No Man Has Gone Before, thus blurring the lines between established roles in the Kirk-Spock-Dehner interaction. But as I said, it's still Star Trek, if only because Kirk manages to get his shirt ripped in the climactic fistfight, thus establishing the tradition of the viewing audience being treated with titillating glimpses of William Shatner's physique in every other episode.