Miri is one of the strongest and most gripping episodes of the original Star Trek, providing a pretty shocking spin on the age-old post-apocalyptic Earth scenario (except for the fact that none of this takes place on Earth, obviously).
The location is a planet which is an exact replica of Earth: same continents, atmosphere, buildings (albeit a bit old-fashioned) and people. Actually, there's a problem with the people: the planet is inhabited exclusively by children. No Stephen King-like twists (à la Children of the Corn), though: as Miri (Kim Darby) explains to Kirk, all the adults were wiped out several years ago by a virus which doesn't affect children. Dr. McCoy quickly comes to the conclusion that the virus works only on individuals who have already reached puberty, and with every crew member of the Enterprise - apart from Spock, as usual - starting to show symptoms, their exploratory mission becomes a race against the clock to find an antidote before someone gets killed, be it by the virus or Miri's more uncooperative "peers".
A lot of science-fiction thrives on the idea of what our planet would be like without specific groups of people. In this case, however, the story serves as a much more potent reflection on a seemingly trivial theme like puberty: a simple plot gimmick like a virus becomes a powerful metaphor, using the opportunities given by the sci-fi context to explore previously uncharted territory and delivering a compelling portrait of young people yearning to be part of the grown-up world even though they aren't quite ready for that yet (this is most obvious in the case of the titular character and her feelings for Kirk). The fact that it's a tense and scary story helps a lot, too.
