Kirk faces off against the Horta. The Horta's eggs are in the background.
Original air date: March 9, 1967
The Enterprise is called to a mining planet, where the personnel are being killed by an invisible creature. Said creature turns out to have a point of view too; she is the Horta, the mother of her race, and the human miners have been carelessly destroying her eggs, thinking they are nothing but a curious mineral formation. These facts are established by Spock through a mind meld. The miners — to their credit — are appalled at what they have unknowingly done, and the Horta is presumably equally distressed to discover the aliens she's been killing are not deliberate child murderers. Happily, both sides are willing to let bygones be bygones and Kirk is able to negotiate a peace treaty and alliance that makes the human miners — and presumably the Horta — "embarrassingly rich".
The Tropes in the Dark:
- Ambiguous Syntax: After the Horta etches "No Kill I" into some rock, Kirk comments to Spock that he wonders if it means that either the Horta doesn't want to be killed, or it's pledging not to kill them.
- Antagonist Title: Subverted. The "Devil in the Dark" turns out to be an intelligent being acting in defense of its brood. The true Devil in the Dark turns out to be the miners who killed the baby Hortas by damaging and destroying the mother Horta's eggs.
- Anti-Villain: The Horta is revealed to be protecting her species' nest, filled with millions of Horta eggs endangered by clueless human miners.
- Arbitrary Skepticism: Spock interacts with aliens that have somehow all evolved to look almost exactly like his own species on a daily basis, and yet it's the idea of a Silicon-based lifeform, something that is indeed scientifically plausible (if thought to be quite rare), that makes him incredulous.
- Being Watched: At one point, while Kirk and Spock are in the underground tunnels hunting the monster, Spock is able to tell when the Horta is watching him and Kirk. It isn't made clear how he knows (possibly Vulcan telepathy).
- Breakout Character: In a way. The Horta became immensely popular with the Trek fanbase, and show up as characters in video games,
Fanon and Expanded Universe literature. - Character Catchphrase: The first ever use of McCoy's "I'm a doctor, not a [XXX]" catchphrase (in this case, "bricklayer").
- Chekhov's Gun: The silicon nodules turn out to be Horta eggs. Their destruction was what caused the conflict.
- Chromosome Casting: Not counting the Horta (depicted as female, but portrayed by male János Prohaska) and a briefly-seen female extra, the episode has an all-male cast. This is the only Star Trek production to have no female speaking parts.
- Dug Too Deep: That's what inadvertently caused the conflict in the first place.
- Eat Dirt, Cheap: The Horta, being a silicon-based lifeform, eats minerals. Its palate is apparently so refined it can easily identify different minerals and ores, making them, as the episode states, "the finest natural miners in the galaxy."
- Easily Forgiven: Once the Horta is able to communicate through Spock's Mind Meld, she is able to tell the miners that they destroyed thousands of her eggs. When she realized they didn't know that, a peace is formed and the Hortae go to work for the miners. Both sides forgive the other for the mistake.
- Fantastically Challenging Patient: The episode has a mining colony terrorized by an unknown creature. Captain Kirk and First Officer Spock explore the mine, and find the creature - a Horta. When it advances, they fire their phasers at it, creating a wound and causing it to retreat. However, once Spock conducts a psychic rapport with the creature, he and Kirk realize it's a brood mother defending her egg clutch. Doctor McCoy is brought in to heal the creature, which he does with silicon-based spackling compound. This ad hoc bandage works well, to the doctor's surprise.
- Green Aesop: What the story becomes when Kirk and Spock learn of the Horta's motivations and the fact that her species is at an extremely tenuous period in their natural history with a very real possibility of extinction because of alien encroachment.
- Hollywood Acid: The Horta dissolves a miner and a security guard, leaving them "burnt to a crisp".
- Hourglass Plot: A compressed example; originally Spock wants to protect the Horta because it's the last of its kind and Kirk intends to kill it because it has been killing dozens of miners and one of the Enterprise's security guards. Over the relatively short time the hunt takes place, Kirk begins to doubt his previous position when the Horta passes up a chance to kill him and Spock becomes more hostile out of protectiveness towards his friend.
- Humans Are the Real Monsters: Downplayed. While the miners earned the Horta's wrath by breaching her nest and destroying hundreds to thousands of her eggs, it was an act of ignorance, not malice. The human miners are appalled when they realize what they've done, and that they've been trying to hunt down an intelligent creature that's only protecting her own home.
- Humans Are Ugly: The Horta calls the humanoid shape "repulsive", though Spock claims Vulcan ears are an exception.
- Hypocrite: Spock attempts to defy Kirk and have the Horta brought in alive, despite the fact that it's been killing lots of miners and could kill more people if they try to capture it rather than shooting to kill. However, when he finds out Kirk is facing it, he starts urging the captain to shoot it immediately. Kirk has to order him not to attack it.
- I'm a Doctor, Not a Placeholder: A bricklayer, in this case. But McCoy still managed to cure the injured Horta, making an Improvised Bandage out of the the silicone concrete used for building emergency shelters.Bones: [quite pleased with himself] By golly, Jim, I'm beginning to think I can cure a rainy day!
- Involuntary Group Split: While Kirk and Spock are hunting the Horta deep underground, the creature causes a collapse that splits them up. After the Horta appears to Kirk, Spock eventually reaches him.
- It Can Think: The Horta steals a piece of machinery from the life-support system, revealing that it's not just a mindless beast. When Spock establishes contact with the Horta, he learns it's highly intelligent and it burns the message "No kill I" on the ground.
- Large Ham: A rare one for Mr. Spock, who goes all out in the mind-meld sequence.
- Last of His Kind: The Horta. Except there are thousands of eggs, some of them hatching by episode's end.
- Let's Split Up, Gang!: With the security guards and miners having headed off in two teams, Kirk and Spock head off together, only for their path to split into two, with tricorder readings indicating the tunnels meet up again further on. Kirk decides they should each investigate one tunnel, despite Spock pointing out the risk in going on alone. Of course, Kirk promptly runs into the Horta; however, his Plot Armor naturally ensures that things don't go as they normally do in these situations.
- Ludicrous Precision: According to Spock, the odds of both he and Kirk being killed in the search for the creature are "2228.7 to 1."
- Mama Bear: The Horta. She’s quite upset that her eggs have been taken and she’s attacking the people responsible for the destroyed eggs. Fortunately, she is a very reasonable one and open to an acceptable compromise.
- A Mind Is a Terrible Thing to Read: Spock feels incredible mental pain when he first tries to establish a Vulcan Mind Link with the Horta. He tells Kirk that she's in agony — she was seriously injured by the earlier phaser blast. He does better on the second try, probably because the creature is more trusting, and he might be using Vulcan mental disciplines to get past the pain (we see him do this in "Operation Annihilate").
- Monster Is a Mommy: The Horta's motive for killing the miners — it's trying to protect its nest.
- More Expendable Than You: Kirk tries to make this argument, saying that while individually both he and Spock can be replaced, if both of them are killed it would be a grievous blow to Starfleet. Spock ignores him.
- Mother of a Thousand Young: Momma Horta. Good thing she and her young reach a peace agreement with the miners.
- My God, What Have I Done?: Both the miners and the Horta react this way when the truth comes out — the miners are horrified to have unknowingly almost-slaughtered an intelligent species, and the Horta is dismayed that it was killing people for a crime they didn't even know they were committing. This helps both sides reach a truce.
- Nice Job Breaking It, Hero: Turns out that monster you shot? It was an alien nanny who was only acting in defense of the nursery. And those weird stones that you've been trying to crack open? They're the babies.
- No-Sell: One of the miners shot the Horta with a hand phaser (Phaser 1), which didn't even slow it down. Kirk surmises that using phaser pistols (Phaser 2) may be more effective, and he and Spock actually inflict damage with their phasers.
- Not Evil, Just Misunderstood: A rare double-sided case. The Horta was mistaken for malicious because it was acting in self defense to protect its eggs. Likewise, the Horta thought the humans were invaders trying to take its eggs, but they were just harmless miners who didn't realize that they were stealing eggs.
- "Not So Different" Remark: When the miners come to kill the creature, Kirk threatens them if they proceed. One of them protests that the Horta has been killing their men, and Kirk retorts that they've killed thousands of her children. The sides' mutual horror at the destruction they've been causing while attempting to protect their own helps them come to an agreement, and the two end up coexisting very happily.
- Not So Stoic: Spock begins to panic when the Horta causes a cave-in near Kirk, and it only heightens when Kirk reveals that the Horta is right in front of him.
- O.O.C. Is Serious Business: Spock, the pacifistic scientist of the group, takes aim at the Last Of Its Kind Horta with a phaser when he catches it near Kirk.
- Off-the-Shelf FX: The unbroken Horta eggs were toy bouncing balls painted gold.
- Red Shirt: The poor schmuck at the beginning, as well as several more before the episode's started. One of Kirk's men also bites the dust.note
- Rock Monster: The Horta, a silicon-based lifeform (at least we think it's supposed to be a living rock). Calling it a "monster," however, turns out to be a misnomer.
- Shaming the Mob: Kirk and Spock tell the Horta's motives to the angry personnel after Spock's mind-meld turns up some interesting facts. The miners, having been ignorant but not malicious, back down.
- Silicon-Based Life: The Horta is a life-form based on silicon instead of carbon. It lays eggs that are almost pure silicon, with a few trace elements.
- Sweet and Sour Grapes: Even after The Reveal, the miners are crestfallen when they realize that the presence of an intelligent lifeform means they'll most likely have to abandon their mining operations. Then Kirk reveals that the Horta is willing to let bygones be bygones and help the miners get access to even more minerals as long as they don't kill any more of her young.
- Tempting Fate: When leaving Schmitter alone on guard duty, Chief Vanderberg assures him "You'll be all right." No, he won't...
- Tunnel King: The Horta, due to the acid secreted from its skin. In Kirk's own words, its species "moves through rock the way we move through air, and it leaves tunnels. The greatest natural miners in the universe."
- What Measure Is a Non-Human?: One of the best episodes to point this out, as the Horta is the most alien-looking lifeform we ever meet on the series... and one of the most popular non-humanoids in the franchise.
- You Called Me "X"; It Must Be Serious: When a cave-in occurs in Kirk's area, Spock begins to call "Jim" rather than "Captain" into the communicator.
- You No Take Candle: When trying to communicate its lack of hostility to Kirk and Spock, the Horta carves "No Kill I" into the rock. The potential double-meaning is acknowledged: either the Horta doesn't want them to kill her, or she doesn't want to kill, but feels she has no alternative.
