Miri
Kirk and his away team contract a disease on a parallel Earth inhabited by children who seemingly never age. McCoy races to find a cure for young and old.
What should Kirk fear more than any other force in the universe? Kids! They give him a solid bonk-bonk here, and they'll be back. There are some interesting ideas offered by the Peter Pan premise, the life prolongation project, and the parallel Earth, but the plot is too contrived to convey real ideas. Kirk and Rand are convincing as they slip into a fevered state; McCoy and Spock, not so much. As for the children, one must wonder if the call sheet specifically stated they must be the most irritating and ugly tykes willing to appear on camera. At least the Kirk-getting-beaten scene is creepy, with the smiling girl. Plus the Captain shouting "No blah blah blah!" is priceless. The captain again proves himself a persuasive speaker. Unfortunately his efforts don't amount to much, and this remains a mediocre installment.
Spock Saves the Day: Spock's research solves the planet's mysteries and aids McCoy in finding an antidote to the disease.
Oops: The children are twice "like mice" - we got it the first time. And why no nerve pinch for the creature? It would have saved Kirk three punches.
Developments: For the first time but not the last, Spock is immune to the Affliction of the Week. He's the only officer to keep his cool, though he also wouldn't take the same risk as McCoy.