Nurse Chapel is reunited with her fiancé Dr. Roger Korby, a brilliant scientist who has "perfected" the ability to create an android copy of a human being, and hopes to replace humanity with these superior, emotionless androids in the interest of removing dangerous emotions from society. Needless to say, Kirk does not agree with Korby's views.
"Little Girls" is a good example of Trekkian ideology. There's a plot here featuring a kidnapping and some attempted escapes, but the story is much more concerned about dialog concerning the nature of existence. For example, would stripping away humanity's ability to feel in favor of cold, strict logic make it less prone for violence and aggression? What about stripping away the positive aspects of emotion, like compassion and generosity? And what happens if you can preserve a person's mind in an android body, potentially forever? And what happens if the androids somehow evolve and create these pesky feelings all over again? Those are some of the intriguing questions posed by the story, though the answers provided only begin to scratch the surface.
There are some moments in the episode that are a tad silly, like a scene where Kirk somehow manages to trick Ruk (Ted Cassidy) into turning against Korby by using some strategic use of logic and semantics ... although the argument Kirk comes up with doesn't really seem to make much sense. Of course, two redshirt deaths also come across as a little pointless, but, hey, that's how clichés are born. (Also, another story so soon after "The Enemy Within" featuring a duplicated Captain Kirk may be pushing it.)