"This Side of Paradise" was partly the work of D.C. Fontana. That and everything else about it gives me plenty of reason to rate it most highly.
And I don't say high lightly.
Tell me that planet isn't hot-boxed.
Old Elias Sandoval had a farm... Omicron Ceti III. And on that farm he had some Berthold Rays.
With no oink oink here, no oink oink there...
No animal life can live here more than a week.
The colonists could not still be alive... except, hey, there's Elias!
Spock is convinced they can't be alive.
"Is it possible that they're not?" Sulu asks.
Spock knew an English girl called Leila Kalomi six years ago on Earth. She's here, now, and in Soft Focus brand overalls!
She and all the other settlers are fine. If everyone was so healthy, McCoy could throw away his shingles. Uh, shingle.
Their scars all disappeared, and everyone's real mellow.
Of course, the animals all died, but they're vegetarians. Problem solved!
Leila takes Spock to a meadow and infects him... uh, with some spores.
"I love you. I can love you." Under the influence of the weed, life seems good.
Spock loses his uniform pretty quickly! I guess he must have taken it off when he and Leila... climbed trees. Clung to lumber. Frolicked in the autumn mist with Puff the Berengarian Dragon.
This planet is a Triffid Grow-Op!
Kirk got a whiff of some of the plant smoke, but he didn't inhale.
McCoy fits right in. Even grew his tonsil's back, sho' nuff.
"Who wants to counteract paradise, Jim boy?"
Somehow, Jim Kirk is the only pooper at this party. With his whole mildly mutinous crew on the space hemp, he can't play spaceship alone.
Then Jim gets a better snootful. He gets it. Packs his groovier green shirt. Puts his antique Samsonite suitcase on the transporter, but gets violently angry at the idea of being happy. He straightens up and flys right.
Kirk passes on his angry good fortune by taunting Spock with racist epithets, sexual innuendo, and suggests the man is a circus freak. He's damn lucky he escaped with just some bruises.
"I don't belong any more." Spock mourns the cure more than the disease.
Kirk has Spock build an auditory irritant, a subsonic transmitter that pisses everybody off. Sobers up all 500 waste-oids.
Spock is his old self: which means he and Leila have to break up.
"If there are self-made purgatories, then we all have to live in them. Mine can be no worse than someone else's."
This is a genuinely heartbreaking moment. Fine work from everybody.
I also like McCoy's line when Sandoval says he's not needed as a doctor anymore:
"You want to see how fast I can put you in a hospital?"
Don't get between a good ol' boy and his mint julep!
I hope they don't just abandon this place for good: provided you don't bring your cats along you can drop deathly ill people here until perfect health is bestowed upon them. It even cures glaucoma! Then show them 10 seconds of 'Toddlers in Tiaras' to get them angry, and head home again. Drink plenty of water. Repeat as needed. Bring snacks if you get the munchies.