The Enterprise receives a SOS signal from a planet that is a near exact duplicate of Earth... expect that all the adults have disappeared, leaving just the kids.
****
In this episode, Kirk punches a 'zombie' three times. This is awesome until you realise exactly what the 'zombie' in question is in an episode with some fairly deep horror quotient when you start thinking about it. In fact, the BBC did not air it again for two decades after initial UK transmission as they felt it was too horrific for broadcast.
In a desolate town, Kirk and his landing party (including Yeoman Rand) encounter a large group of children who have no proper parental figures, for whom grown-ups, or "Grups" are objects of fear. When they discover what caused the adults to go, we go into a fairly standard race against time plot with one rather big added complication, There is a scene in a cob-web filled where Kirk has to persuade these rather feral children (a number of these were played by relations of the cast, including Shatner's then five year old daughter Lisabeth, who says she was fascinated by Rand's hairdo) that he offers the best hope of their salvation... while they're prepared to attack him. My thoughts were "welcome to a British inner-city school", but that might just be the inner cynic in me.
Chief among these is the titular Miri (Kim Darby, who was 19 at time of filming, her costume designed to hide her fuller adult figure), a young lady on the cusp of womanhood, who, quelle surprise, falls for Kirk. Now, I doubt she was the only teenager who had a crush on Shatner, but what slightly throws me is Kirk telling her that she's very pretty in response. The line between dealing with a highly emotional kid and 'grooming' is a thin one... I must admit to being slightly uncomfortable with those scenes.
Kirk gets to do some shouting, Spock gets to be very logical and McCoy commits medical malpractice on himself in an episode which firms up the key archetypes of the leads.
While there is a good deal of horror stuff just below the surface, there are also plot holes big enough to fly a Galaxy-class starship through. The event that eliminates the adult population is such that is very unlikely most of the children would survive for as long as said in this episode. Also, identical down to the continents? Really? That smacks of trying to save money with stock imagery with an episode already filmed in a backlot - namely 40 Acres, which was used for The Andy Griffith Show and also Gone with the Wind.
Conclusion
An entertaining and atmospheric episode, but does fall apart a bit if you look at it too closely.
8/10