"The Devil in the Dark" is a first season episode Star Trek: The Original Series which aired on March 9, 1967. It is episode #26 and written by Gene L. Coon and directed by Joseph Pevney .

On stardate 3196.1, the Starship Enterprise, under the command of Captain James T. Kirk, is sent to the pergium mining colony on planet Janus VI to investigate reports of a strange creature that has killed nearly 50 miners and is destroying equipment.

Soon after their arrival, the Enterprise sends down a landing party, and they discover that the killings started soon after a new level, #23 was opened. Corpses found in the area were smoldering heaps of liquified flesh and bone. Mr. Spock speculates on the killer; a being of great stealth and speed, who attacks mercilessly, and nearly disintegrates it's victim with a powerful acidic substance.

Soon, another attack is reported in which a guard is killed, and a circulating pump vital to the colony's main reactor is stolen. The reactor equipment is found to be so old and outdated that the Enterprise has nothing to replace it in the ship's stores. Engineer Montgomery Scott manages to jury-rig a temporary replacement, however he says the original component must be found or the life support systems will fail, threatening the lives of everyone in the colony. Spock says, whatever the creature may be, it has extraordinary intelligence, and knew just how to damage the colony. Questions arise that it may all be an inside job, that perhaps one of the miners is homicidal. However, evidence points to something else that is lurking out there.

Kirk and Spock lead a security team deep into level 23 to locate the saboteur. They soon encounter a rock-like creature which appears like a animated clump of molten stone. The beast threatens Kirk and survives a direct phaser blast. Injured, the creature quickly escapes by burrowing directly through a solid rock wall. Upon surviving the blast, Spock believes the creature is a silicon based lifeform , not carbon based and is resistant to the older "Type I" phasers weapons carried by the colony guards, however the landing party's "Type II" phasers should be able to stop it.

Spock also determines the acid the creature secretes is so powerful, that it melts rock on contact, and it can move through rock like a fish moves through water. They also encounter numerous silvery spheres laying about that they have never seen before. When questioned about the spheres, the mining colony leader, Chief Engineer Vanderberg , describes them as silicon nodules, a useless, but interesting mineral formation. Vandenberg, in fact, keeps one displayed on his office desk. Vandenberg also begins to pressure Kirk to get rid of the creature immediately, instead of bumbling around with unrelated subjects, and if Kirk doesn't take care of the problem, he will.

Investigating the fresh tunnel patterns, Spock figures that there is only one creature, and thinks it may be unique. He warns that killing it would be a scientific loss. Captain Kirk thinks otherwise, explaining that creature is too dangerous and must be stopped at all cost. Spock interjects, but Kirk cuts him off and tells him it would be better if he goes to supervise the repairs on the reactor. Kirk explains the reason for the split up is, in a worst case scenario, he doesn't want his second in command killed along with himself. Spock reassures him, calculating the odds of both of them being killed at 2228.7 to 1.

Next, Kirk discovers a chamber filled with thousands of the silicon spheres along with the missing component to the reactor. The roof suddenly collapses separating Kirk from Spock. Kirk is trapped inside the creature's lair and it appears from the shadows ready to lunge at him. Kirk draws his phaser and to his surprise, the creature suddenly cowers back. Kirk tests the beast's response; lowering the phaser the creature advances, when he raises, it cowers back again. Kirk, now intrigued with the stand off, sits down and begins to "talk" to it.

Spock makes contact with the Captain who explains his situation and Spock finds another passage that leads to the scene. When he arrives, Kirk tells Spock to make no sudden moves. As Spock analyses with the tricorder, the creature turns, showing them a large open wound in its side, and then climbs upon a rock in which it scrawls out the words: "NO KILL I".

Spock and Kirk are fascinated that the creature shows this kind of intelligence, and Spock decides to mind meld with it. In doing so he feels the intense pain of the creatures injury and cries out, however he keeps contact with the beast and discovers more information. The creature calls itself a Horta and that it is the last caretaker of the silicon spheres which are revealed to be its eggs. The creature says it is "killing the murderers", the miners who were "destroying its children".

Sympathetic, Kirk orders McCoy to come to their location and help the creature. When McCoy arrives and analyzes the Horta's obscure physiology, he doesn't know where to even begin saying "Damnit Jim, I'm a doctor, not a bricklayer!" He patches up the wound with quick setting plaster, that he orders beamed down to the bewilderment of the supply officer; "Never mind what I want it for, just beam it down!".

Meanwhile, Chief Vandenberg and the other miner's patience wears thin. They decide to take matters into their own hands and deal with the creature once and for all. A mob forms and overpowers the security team that was to keep the miners out of the way. The mob makes their way down to level 23 and arrive at the lair. The mob threatens to kill the creature and the landing party if they intervene, but Spock stands to address them. He explains that the Horta was just protecting it's eggs, the worthless silicon nodules the miners have been collecting and destroying. The creature has a 10,000 year reproductive cycle. When they die out, one stays behind to look after the eggs, and when the miners broke into the egg chamber the creature had to defend the offspring which are in fact, ready to hatch.

Vandenberg gets concerned that there will be thousands of those "things" running around, but Kirk explains the Horta are peaceful. They simply eat rock, and are very efficient tunnel makers. They could actually work with the miners, helping them make tunnels to wealthy deposits of minerals. When Vandenberg realizes the potential, his eyes light up. He decides to agree to call off the mob.

As the Enterprise prepares to leave, Vanderberg informs Kirk than he and his men are getting along well with the hatching Hortas, although adjusting to their appearance is a little difficult. Spock mentions to Kirk that the Horta found human appearance repulsive, however the Horta apparently liked his pointed ears.