The gang beams down to a planet identical the Earth in the 1960s, although I honestly think it looks more like the old west. The planet is populated entirely by children due to a virus created by the people living there. The virus makes children age at the rate of one month for every 100 years, although because of a slight miscalculation on the part of the scientists who made it, it also turns all adults into ravenous scab-monsters who quickly die. Naturally, the Enterprise crew lacks the foresight to check for stuff like this before they beam down, and they are all immediately infected.
The crew befriends a young woman named Miri, who apparently has the hots for Kirk. Kirk doesn’t do anything to diminish the affections of the underaged girl by touching her face and giving his usual creepy smile. As the crew begins exhibiting the death scabs, Spock and McCoy work frantically to find a cure.
The mischievous kids on the planet distract Spock and McCoy and swipe their communicators, which have all been left laying out. Without a way to communicate with the ship, the crew has lost the ability to test their serum. Even I know not to let your cell phone out of your sight around kids; they always want to play Angry Birds on it.
Meanwhile, scorned by Kirk’s affection towards Yeoman Rand, Miri has betrayed Kirk and led him into an ambush with the rest of the children on the planet. The kids proceed to beat the crap out of him, but not before he throws a few of them around violently and gives his regular speech. The kids obnoxiously yell, “Bonk bonk” as they pummel Kirk with hammers. It’s kind of baffling how the kids have managed to stay so childish, despite having lived for centuries.
At last, Kirk manages to reason with the kids and McCoy develops a serum by testing it on himself. The enterprise leaves a medical crew to look after the children and takes off. Nobody even questions why there’s a duplicate version of Earth in existence.