According to Memory Alpha's In Universe Timeline, the story sequence is:
A Taste of Armageddon : The Devil in the Dark : Errand of Mercy.
In the Remastered Episodes Chronology, the release sequence is
Miri : The Devil in the Dark : The Naked Time.

Summary

On Janus VI a "monster" has killed over fifty workers of a pergium mine, burning them to a crisp. Just as the Enterprise landing party has arrived, it strikes again and steals a vital pump from the colony's power plant. Spock suspects that the creature, which is capable of moving through solid rock, may be a lifeform based on silicon, and he advises against killing it. Then the creature appears right in front of him and Kirk. They shoot and severely hurt it before it retreats through a tunnel.

Kirk pursues and finds the creature, but he discovers that it is afraid of the phaser and so he doesn't shoot. While McCoy treats the creature's "wound" with concrete and Kirk retrieves the pump, Spock finds out in a mind-meld that the Horta, so its name, is the last of its kind. The Horta was protecting the future generation in the form of silicon "eggs" that the miners have been destroying all along. The miners, led by administrator Vanderberg, appear on the scene to kill the creature. But Kirk holds them back, explaining that they could co-exist with the Horta in a peaceful way and to their mutual benefit.

Errors and Explanations

The Nitpicker's Guide for Classic Trekkers

Plot Oversights

  1. How could an elliptical creature like the Horta create circular tunnels? The creature probably switches shape from elliptical to circular when digging the tunnels.

Equipment Oddities

  1. McCoy using thermal concrete, designed for the construction of emergency shelters, to heal the Horta, but no one sending some down to Sulu and his men when they were trapped on Alpha 177 during 'The Enemy Within'. They probably didn't have it in sufficient quantities - if they did, most methods of transporting the stuff to the surface could just as easily retrieved the trapped men. Besides, in those frozen conditions, it might not have been useable!

Nit Central

  1. Keith Alan Morgan on Thursday, April 15, 1999 - 6:38 am: The colonists are armed with Phaser 1, while the ship has Phaser 2. What kind of Phasers did Captain Pike have? Phaser 0? Phaser point 5? Charles Cabe (Ccabe) on Thursday, April 15, 1999 - 12:59 pm: Pike used lasers on his ship. Phasers were invented c.2200 but didn't come into use until Kirk's era.
  2. McCoy says that a silicon lifeform would be impossible in an oxygen environment. However, since no silicon lifeforms had yet been discovered, how could McCoy be sure of that statement? LUIGI NOVI on Sunday, May 27, 2001 - 1:30 am: Prediction based on known scientific principles. There's never been an invisible person, but we know an invisible person would be blind, because the light needs to be absorbed by the eye. Being invisible, the eye would be transparent, and light would pass through it (Unless the person had the ability to control which part of his eyes he made invisible, and kept his eyes visible, though this might defeat any practical purpose of it.) A lot of nitpicking uses such theoretical projection and theorizing. [N 1]
  3. When the Horta appears the wall glows red and fades away like it was hit with an energy weapon, not an acid. ScottN on Saturday, October 14, 2000 - 11:39 am: It could be a highly exothermic chemical reaction, causing the wall to glow red.
  4. How long do you think that thermal concrete bandage will survive when the Horta starts secreting acid to burn through rock? Long enough for the wound to heal?
  5. John A. Lang on Tuesday, October 02, 2001 - 7:17 pm: At one point, Spock says, "Mr. Scott has far more knowledge about nuclear reactors than I do." Seeing Spock is a Vulcan...a superior race to humans...does this statement make any sense at all? KAM on Wednesday, October 03, 2001 - 3:35 am: Spock may be more intelligent, but he doesn't necessarily know everything & it is possible that 'primitive' Earth technology like nuclear reactors may not have been a field that Spock specialized in. Certainly not to the degree that Scotty presumably did.

Notes

  1. See my response on this episode's Nit Central board.


The Original Series Season 1
Pilot episodes: The Cage I Where No Man Has Gone Before
Regular episodes: The Corbomite Maneuver I Mudd's Women I The Enemy Within I The Man Trap I The Naked Time I Charlie X I Balance of Terror I What Are Little Girls Made Of? I Dagger of the Mind I Miri I The Conscience of the King I The Galileo Seven I Court Martial I The Menagerie Part 1 I The Menagerie Part 2 I Shore Leave I The Squire of Gothos I Arena I The Alternative Factor I Tomorrow Is Yesterday I The Return of the Archons I A Taste of Armageddon I Space Seed I This Side of Paradise I The Devil in the Dark I Errand of Mercy I The City on the Edge of Forever I Operation: Annihilate!
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