8. Mirror, Mirror
Mirror, Mirror is best known for the evil Spock, the one with the goatee. There was something about the character, and the way Nimoy portrayed him, that resonated with fans. Its fascinating to recognize that Mirror Spock is not that different than good Spock. Both are fiercely logical, pragmatic, yet each proceeds from a radically different world (galaxy?) view. The idea of parallel universes is explored in this episode. The landing crew is transported to the Enterprise of a parallel universe, a universe where each character has a duplicate and the Federation is a force of terror in the galaxy called the Empire. In Mirror, Mirror, Star Trek explored the ideas behind quantum theory and mechanics long before the subject was tackled seriously by physicists. While the mirror universe was revisited again in both Star Trek: Deep Space Nine and Enterprise, many books, television shows and movies have dealt with the idea of parallel realities to the point where it has become a staple of the science fiction genre.
Memorable line: "You're a man of integrity in both universes, Mister Spock." - Kirk