What this episode is really about, though, is Spock replacing Mitchell as Kirk's companion. The first thing we see is Kirk and Spock playing 3-D chess together and bantering. Spock is almost smiling. Later, when Mitchell joins them in the turbolift,
After Mitchell gets zapped by the Apotheosis Field, it's Spock who is suspicious and monitors his actitivies. And easily convinces Kirk to put a 24 hour guard on Mitchell. Kirk never seems to question Spock's judgement. He does suggest that Spock is rather unfeeling, but Spock replies that that makes his observations untainted. Kirk doesn't really disagree.
In fact, I was surprised by how easily Kirk let go of his friend. I would have expected him to resist a little, to believe that Mitchell could fight the changes and retain a good heart. But he not only lets Mitchell go, he kills him himself and ends the episode beside Spock, sharing a special moment.
It's all true, boys and girls, it's all true.
Also, Spock had a really cool ray gun.