The biggest mystery about this planet is never addressed in any way whatsoever after they beam down. It is an extreme example of What Happened To The Mouse? If this episode had been made today, you can bet the Internet would have lit up with questions the second the closing credits began.
Perhaps they dropped it because there's so much other stuff going on in this episode. There are subplots about Miri being attracted to Kirk, a whole bunch of other children who have somehow been living for 300 years but still act like kids, and the infected crew getting irritable and short-tempered with each other - none of which I could squeeze into these 12 panels.
This strip is the second (future) cross-over with Shaenon Garrity's X-Files recap comic, Monster of the Week. Miri is played by Kim Darby, who would later go on to play Kathy Lee Tencate in The X-Files seventh season episode "Sein und Zeit".
Captain's log, Stardate 2713.5. We've found a distant planet which is an exact duplicate of Earth. We must investigate this astonishing mystery.
Kirk: Let's beam down to the analogue of the Universal Studios backlot.
Caption: A man attacks!
Kirk: Hold him so I can punch his face!
Spock: I could nerve pinch him.
SFX: Punch! Smak! Pow!
Kirk: We're not here to hurt you.
McCoy: He's dead, Jim. It's amazing. His metabolic rate is incredibly high.
Spock: That is incredible for a dead man.
Miri: I'm Miri, and you're evil Grups.
Yeoman Rand: Grups? Grown-ups?
Miri: The Grups all died.
McCoy: A PLAGUE! Awesome!
McCoy: Quarantine conditions! Beam down a medi-lab! I'm gonna cook us up some anty-dote!
Miri: You're very handsome.
Kirk: My name's Jim. And you're very pretty, Miri.
Spock: The plague kills adults. And children entering puberty.
McCoy: Stop doing my PLAGUE work for me!
Spock: Miri will die soon. We'll all die soon.
McCoy: Awww! I wanted to conclude that!
Yeoman Rand: I'm scared, Captain.
Kirk: There, there, Yeoman.
Caption: Subplot: Miri gets jealous and helps other children steal communicators. Oh, and the children all turn out to be three hundred years old.
McCoy: I have an antidote. It's the nitrogen cycle.
Spock: That makes no sense.
McCoy: Who's the PLAGUE doctor here?
McCoy: I'll test the PLAGUE antidote on myself!
Spock: No, it could be fatal. Now, I'll leave you alone with it while I check on the Captain.
Caption: But the antidote works!
Spock: He injected himself. Who will understand the medical mind...?
McCoy: The PLAGUE medical mind!
Yeoman Rand: Miri loved you, Captain.
Kirk: I never get involved with older women. Full ahead, Mr Spock. Warp factor one.
Spock: What about the duplicate Earth?