The crew discovers that the members of a research colony have been killed by an unknown force, the only survivors being their children, who are remarkably devoid of grief over their parents' deaths. Once beamed aboard the Enterprise, these children exhibit a bizarre power over the crew, inducing hallucination and mental incapacitation in their victims. It turns out that an alien entity named Gorgan (Melvin Belii) is using the children to hijack the starship so he can consume more worlds and cause more death for his own evil purposes. Bwahahaha.
"And the Children Shall Lead"—that is, lead this episode straight into the gutter. A dismally frustrating, repetitive, and bland hour, we're forced to watch five children take over the ship in extremely uninteresting ways, making the crew look helpless (who wants to see that, anyway?). The "magic pump-fist action" gets really, really goofy, becoming what feels like a lame parlor trick. Melvin Belii is terrible as the completely uninteresting Gorgan, reciting his lines like a robot. Meanwhile, Bones has the thankless role of telling Kirk every five or ten minutes that these children have to be treated with kid gloves in order to prevent psychological damage. Funny, you'd think their helping an evil alien that murdered their parents might already have done that.
The one potentially promising aspect of the episode—playing off the crew's fears—is not utilized the least bit effectively. This is an episode created on autopilot, lacking any and all sense of inspiration. At least "Spock's Brain's" level of badness made it somewhat amusing. This episode is simply dull to the point of being unwatchable.