To start, I just want to say I really like this episode. “The Devil in the Dark” is actually one of the first Star Trek episodes I distinctly remember seeing as a kid. The Horta may not be the most impressive alien by modern standards, but it’s a memorable one. I say that as someone who was a jumpy kid that monsters easily spooked. As an adult, it’s obviously a guy under a decorated rubber sheet or something.
Still, the creature moves around well and provides a lot of mystery.
As for plot, something is attacking the miners of Janus VI. They mine pergium there, one of those rare, super-valuable sci-fi elements shows like Star Trek use all the time. Whatever it is, the thing leaves nothing behind but heavily burned corpses. It also tunnels through rock easily and can shrug off phasers without any trouble. Fortunately for the colony, the Enterprise is swinging by to investigate.
So, the set-up here is very much a standard sci-fi monster of the week story. There’s a monster killing the miners. Now Captain Kirk and Co. need to find a way to stop it. The miners don’t seem to care what it is or what it does. They just want it gone, preferably dead.
But Star Trek never plays it that way. Even the rare action-oriented episodes tend to have something deeper going on. And while it is true that there are plenty of good shows that would make use of the monster-of-the-week trope as something for the protagonists to somehow defeat and forget about, that isn’t Star Trek. So, we should know from the get-go that there has to be more to this creature than the miners tell Kirk and Spock about.
What clues do we get? First, we see the creature actually breaks into the mining complex and steals a part to the nuclear reactor. That tells us this isn’t a mindless monster. It also puts a ticking clock into the narrative. Scotty can build a temporary replacement, but the word there is “temporary”.
Additionally, Spock asks the head miner about these orb things made, we’re told, of pure silicon. The miner dismisses the things. They’re all over the place and worthless.
Now, the episode does two things I think are especially noteworthy. One is a little weird and doesn’t make a lot of sense as I think about it. The other is pure Star Trek. The weird one is a change in philosophical position. Kirk spends much of the episode acting as a more military-style commander. He’s giving his security people orders. He’s trying to send Spock back out of danger before Spock points out the odds of either of them finding trouble are slim. And it’s Spock who theorizes what the creature is, pointing out that if they do succeed in killing the creature with their revamped phasers, they may be causing a scientific catastrophe by causing the extinction of an otherwise unknown lifeform.
Yet, when the creature comes out in the end of the episode, Kirk is the one who resists the urge to shoot the creature. Spock is more willing to kill it. True, Spock was the one who most vehemently insisted on killing the salt vampire, but it does seem weird he’d be the one most likely to cause the catastrophe he warned against earlier. It’s also true that Spock comes around to Kirk’s way of thinking fairly quickly, but it was still Kirk who got there first.
As for the more Star Trek-style development, that comes with knowledge on what the creature is. Spock does a mind meld with it, discovering that it’s intelligent and calls itself a Horta. Yes, there were plenty of hints up until then, and the show pays all of them off. Those orbs? Those were the Horta’s eggs. It was defending its nest, and Spock and Kirk did manage to badly injure the Horta earlier. McCoy ends up treating it with cement. Maybe he’s a doctor and a bricklayer. The Horta were the native lifeforms for the planet, spending long periods of time in a form of hibernation. And when the miners storm in to finish what Kirk started, he tells them this.
And they realize what they’ve done and feel bad.
A lot of lesser shows would have gone a different way than this. Some of the miners might have still wanted revenge, or refused to believe Kirk and Spock. They might have attacked the Horta, or the Horta might have moved again in self-defense, forcing the creature’s death.
That is what happened with the aforementioned salt vampire after all.
But no, not only do the miners feel contrition, but the Horta and the humans come to an arrangement. The Horta and her children will dig tunnels to valuable minerals, and the miners will dig them out. Everyone lives in peace and gets what they want. Heck, the Horta even shows where the nuclear reactor’s missing part is. The whole thing was mostly done out of ignorance, mostly from the humans but the Horta wasn’t entirely innocent either. They were just unable to talk to each other.
Ironically, both humans and Horta found the other hideous. Spock is somewhat proud to point out that the Horta did like his ears, despite Kirk and McCoy’s general teasing. His pride sure does seem human, but there’s probably no reason to be insulting to Spock over stuff like that. I mean, he did put his hands on a silicon thing that can easily swim through rock. That’s a level of bravery that the others didn’t show right away.