He doesn’t kill the plants; he and Spock just devise a subsonic set-up to irritate the hell out of the colonists. Once they were pissed off, they started fighting among themselves and were able to overcome the “feel-good” effect of the plants’ spores.
“The Empath” and “Plato’s Children” are worse still, IMHO. We should have a poll on which Third Season episode of TOS is the worst.
“Bonk, bonk on the head!” was in “Miri,” not “And the Children Shall Lead.”
In TNG, they had the first, but still not the second.
What exactly was the problem with the plants? Spock was actually happy! Or did he just think he was happy? Were the plants going to eventually kill them or something?
Nitpick: I believe the correct British spelling is "gaols.’ Goals are what football players score.
The effect of the spores was like a drug. Spock, McCoy, Sulu, and eventually Kirk were high on the “happiness” the spores created, just like the colonists.
The message was “Drugs stifle creativity and progress, so they’re bad.”
The last surviving Logitech game controller was used in the recreation of the Titan submersible, for the Museum of Immense Stupidity.
The Alternative Factor. At least Space Hippies and Brain and Brain have actual stories that make sense. And, it’s not even a third season episode.
In actual fact, there was nothing wrong and everything right. The planet should be a destination hospital. Got a broken spine? Missing spleen? A bad case of Xenopolycythemia? Want your tonsils back? Go there, get dosed, get magically repaired, and go into the anger booth to remove the spores’ influence, and you’re right as rain. Please exit through the gift shop, and thank you for coming!
Not a Third Season episode. It came near the end of the First, IIRC.
Both are in running. Good idea.
Most episodic shows never came back to cool stuff. Same with Shore leave/
The animated series did. And IMO got it completely wrong. There is no caretaker! It’s all automated. The planet is designed to give you whatever you want. And what the crew wanted was an answer. So the planet gave them one. How dumb were they to think one lonely old guy sat waiting on this planet, this large earth-sized planet, all by himself, just waiting for something to happen? BOOORRR-ing.
But to your point, we never saw the people that made this planet. No one owned up to it. Were they all dead, like the makers of the Guardian, or those world-building Kalandans that accidentally killed themselves? And the planet is just running on automatic…forever?
Still is the worst episode. Being a first season is “bonus points” in the ranking. They have no excuse!
Should be “Stepchildren.” Mea culpa! 
I thought he’d just died in the intervening years. 
As I recall, in the novelization they find his gravestone, which identified him as the last of his race.
In TAS too.
Interpol not only has hundreds of armed agents they also have jurisdiction in the United States to carry out arrests.
I can’t recall any of my teachers or professors ever addressing the class as “class.”
It’s sort of like films like Interstellar or Project Hail Mary. Like no one ever mentions again how humanity in it’s desperation just sort of happened to harness interstellar travel and planetary-scale engineering?
Here’s something that almost never happens anywhere except for rom-coms.
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Your entire career is suddenly dependent on landing that one project/client you only have a few days to finish (that inexplicably coincides with some other activity that, objectively speaking, usually isn’t that important)
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In the 12 to 72 hours it takes for the man do whatever hairbrained scheme he is working on, his wife/spouse and made a decision to move out, found a new place to move to, and is just about to finish packing by the time he shows up to mend the relationship.
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Man runs to his woman and exclaims how he quit/gave up his job for her because he loves her so much! Because so many women want to be with a dude with no job!
Which is yet another example of “screenwriters thinking all jobs work like screenwriting.”