Kirk faces off against the horta. The horta's eggs are in the background.
is called to a mining planet, where the personnel are being killed by an odd creature. Kirk at first determines to kill the creature, but after discovering that it is not only sentient but also well-meaning, he is able to negotiate a peace treaty.
- Antagonist Title: Subverted.
- Anti-Villain: The Horta is revealed to be protecting her species' nest, filled with millions of Horta eggs endangered by clueless human miners.
- Chekhov's Gun: The silicon nodules turn out to be Horta eggs. Their destruction was what caused the conflict.
- Dug Too Deep: That's what inadvertently caused the conflict in the first place.
- Eat Dirt Cheap: The Horta.
- I'm a Doctor, Not a Placeholder: A bricklayer, in this case. But McCoy still managed to cure the injured Horta.
- It Can Think: the Horta steals a piece of machinery from the life-support system, revealing that it's not just a mindless beast.
- Last Of Its Kind: the Horta. Except there are thousands of eggs, some of them hatching by episode's end.
- Mama Bear: the Horta.
- Monster Is a Mommy: the Horta's motive for killing the miners.
- 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.
- OOC 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.
- Rock Monster: The Horta, a silicon-based lifeform.
- Shaming the Mob: Kirk and Spock do this to the angry personnel after Spock's mind-meld turns up some interesting facts.
- Silicon-Based Life: The Horta.
- Throw It In: The whole episode. Acrobat/stunt actor Janos Prohaska was promised an episode if he could come up with a convincing alien. He demonstrated the Horta outfit in producer Gene Coon's office and Gene immediately signed off on using it.
- Tunnel King: the Horta, due to the acid secreted from its skin.
- 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.
- 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.