The plot is straightforward: Enterprise is called to Janus VI, a long established mining colony that supplies critical minerals to nearby worlds. After 50 years of uneventful and profitable mining, something begins killing the colonists, dissolving them with a powerful, corrosive acid. The miners' weapons are ineffective, and vital shipments are not going out. Kirk, Spock and McCoy beam down to investigate. Spock quickly figures out that the most likely agent of the attacks is a silicon-based lifeform that moves through rock like humans (or Vulcans) move through air. The only puzzle piece missing is "why?" Why, after a half-century, has the creature begun to attack the miners? Before any progress can be made answering that question, the alien steals the coolant pump of Janus' antiquated fusion reactor, and Kirk faces the utter destruction of the colony if he can't get it back. So Enterprise's entire security detail, Kirk and Spock begin combing the shafts, hunting the creature. Naturally, it's Kirk who stumbles across it but oddly it doesn't immediately attack him. Instead it shows him a wound caused by an earlier phaser shot, and appears to want something. Spock joins Kirk and uses the Vulcan mind meld to communicate with the creature. In the meld, he learns that the alien is a "horta," the last survivor of the previous generation of horta and the guardian of the eggs of the next. It turns out that the miners had recently breached the incubation chambers and were killing thousands; the horta was only trying to protect her children. There's a minor contretemps when the colonists overwhelm the security men and try to kill the alien but when Kirk explains the situation both sides come to a modus vivendi where the horta would live as they had always done, the humans would follow and use the resulting tunnels to extract the ores, and both would share in the profits. All things end happily for all concerned.
Or - realistically - would they?
"The Devil in the Dark" is one of Star Trek's best episodes but it raises some disturbing questions if thought about too deeply.
The first is the question of how "friendly" the horta is. Her change of attitude and good will comes suspiciously fast. Perhaps, having shared minds with Spock, she realized how outclassed she was and how easily Starfleet could wipe out her entire species. She doesn't make nice with the colonists because she realizes what a swell bunch of guys they are but because she's terrified that they'll slaughter her and her children if she doesn't.
And what of the future? The horta are not a hive mind - one consciousness, many bodies. It's hard to imagine that every newborn will accept their mother's solution to the "human infestation." Is is not possible that an insurgency could spring up dedicated to finishing what mom had started? And how would the Federation respond? Or let's consider this: A nonviolent movement arises among the horta that wants the humans off world. It gains enough support so that the Federation is formally asked to leave. How would the UFP respond to that? If the Federation were to live up to its ideals of freedom and noninterference then the answer is obvious: They'd leave. But, as we know all too well in the US, a country's founding ideals often founder on the shores of so-called vital interests and national security. After 50+ years, the interests vested in keeping the Janus mines open would exert enormous pressure on Starfleet to keep them open no matter what. And there's the horta who are OK with humans on Janus. Would Starfleet provide them with aid to suppress the anti-human faction? I can easily see the mining lobby explaining to the Federation Council that the anti-human horta don't represent the species as a whole and have no legitimacy. That Starfleet needs to aid the pro-human side because they're being oppressed.
And let's not even begin to think about what kind of consciousness a silicon-based lifeform that looks like this:
would be like. On its face, to assume the horta perceive and think like humans and Vulcans is absurd.
And what are the effects of a human presence on a silicon-based ecology? Assuming the horta are the only lifeforms is another absurdity never addressed by the episode, there must be a host of other creatures moving through the rock.
The point - if I can dignify this posting with a purpose - is that I've been thinking lately about the survivability of civilization-as-we-know-it and coming to some pretty gloomy conclusions. "The Devil in the Dark" is an example of the exploitative, extractive, consuming-all attitude that characterizes our culture. There's no thought to the long-term consequences of our appetites in our zeal to satisfy our short-term needs, and now that we're running up against the limits of our planet, we're beginning to see the price we and our descendants are going to pay.
Alas, for the days of my youth when Star Trek was just plain old fun to watch.