The plot is way out there (an alien that is "pure energy" stealing the bodies of the crew). This episode is guilty of a lot of deus ex machine considering the amount of the story that is not at all plausible. However the acting is good, and it's interesting to see an evil Spock and to hear the echoing voices. The ending is kind of "cute" as well. It also suggests the reason why all aliens in Star Trek have humanoid form, that their race had spread it's genes over the galaxy. I enjoyed the theme of the idea that power (or rather, arrogance) is dangerous and devalues life. I think it was one of the better episodes of the series.
Review of Return to Tomorrow
