There's a lot going on in this little episode. You've got Kirk versus a bunch of children, the crew of the Enterprise against time, and a little love triangle action all wrapped up inside a parallel Earth tale. It earns points for ambition, and for not dropping any of those balls entirely, but by the same token, none of those various strands fully deliver the goods in the end. For instance, the idea of a parallel Earth is plenty interesting, but it fails to achieve any real significance in the narrative. And however pragmatic it may have been, Kirk's ease at manipulating Miri's affections is a little creepy, although it's interestingly subverted when he discovers that she's actually far older than he is. The crew's efforts to discover a cure probably constitute the most successful of these plot threads, but even that is undercut somewhat when their efforts to retrieve the communicators from the Onlies are proven irrelevant by McCoy's last-minute gambit. Finally, not much is made of the potentially interesting conflict between Kirk and the Onlies. Since the Onlies revel in chaos, reasoning with them neither rings true as their motivator nor makes them a unique antagonist for the Captain.
Still, one must give credit for the amount of food for thought on offer, even if most of it has been only half baked.
Still, one must give credit for the amount of food for thought on offer, even if most of it has been only half baked.