(written from a Production point of view)
The Enterprise arrives at Janus VI, where an unknown monster is destroying machinery and killing the miners, threatening the entire mining operation.
On Janus VI, a miner named Schmitter replaces Sam at his guard post. Schmitter is extremely nervous about doing guard detail, but Chief Vanderberg reassures him by recommending that he have his phaser on him at all times and if he hears or sees anything, help can arrive in three minutes. Moments after Chief Vanderberg and the other guards move on, they hear a scream and return to find Schmitter reduced to a pile of ashes.
Two days later, the USS Enterprise arrives at Janus VI in response to the distress call. For the past three months, a creature has been terrorizing the mining colony, killing more than fifty people, sabotaging machinery on the lower levels, and bringing the pergium production to a halt.
Captain Kirk, Spock, and McCoy analyze the situation in Vanderberg's office. Only one man saw the creature and lived: Chief Engineer Ed Appel, who shot it with his phaser type-1 with no effect. He doubts that the power of a starship will do any good "down in the tunnels." McCoy analyzes Schmitter's remains; they are not burned but corroded, as if he had been immersed in a vat of extremely corrosive acid. Spock is fascinated by a silicon sphere on Vanderburg's desk; Vanderburg says there are millions of them underground but they have no value.
Their conversation is interrupted by an alarm at the colony's nuclear reactor. The creature has killed the guard outside, burned its way in, and taken the reactor's main circulating pump, without which the reactor will go super-critical and irradiate half the planet.
Scott reports from the Enterprise that he can rig up a replacement with "some odds and ends" that might last 48 hours. Kirk orders him to beam down with it and hopes that, within that time, the original pump can be retrieved.
Spock, still contemplating Vanderburg's sphere, speculates that they may be dealing with a silicon-based lifeform rather than carbon-based life. This would explain why the creature does not show up on sensors and why it was impervious to Appel's phaser. Kirk summons Lieutenant Commander Giotto and a security team, and Spock adjusts their type 2 phasers to be more effective against silicon. The security team is dispatched to level 23, which was opened just before the attacks began. Kirk orders them to set their phasers to maximum, and to fire whether or not attacked.
A security officer is killed by the creature, bringing Kirk and Spock to the scene. They see the creature, and fire on it, damaging it, but it gets away, tunneling through the rock with its acid. They examine a piece of the creature, which seems to prove Spock's theory of silicon-based life. Spock detects only one such creature within a hundred miles, and suggests that to kill it would result in the extinction of the species, "a crime against science," he states. Kirk says there is no alternative.
The search teams focus on where the creature was seen. Spock gives them orders to "surround it, and possibly capture it" but Kirk countermands him, stressing that the goal is to "shoot to kill." Kirk orders Spock away to assist Scott with the reactor, but Spock counters with logic and Kirk relents. Spock states that the probability of he and Kirk both getting killed by the creature is approximately "2,228.7 to 1," to which Kirk ultimately allows Spock to stay with him. Scott's improvised pump fails and Kirk prepares to beam the miners up to the ship. But Vanderburg and his men refuse to be chased from the planet; they vow to fight the creature – "with clubs" if there are not enough phasers.
Kirk and Spock separate and Kirk finds a chamber with hundreds of the silicon spheres. Spock warns Kirk not to damage them but is still not ready to state his theory about them.
A roof near Kirk collapses, which Spock finds much too odd to be a coincidence. The only way out for Kirk is to continue forward – to a face-to-face encounter with the creature.
Kirk senses the creature's intelligence, as it backs off when Kirk raises and lowers his phaser and it displays its wound from the previous encounter, and Kirk's determination to kill the creature softens. Spock arrives and initiates a Vulcan mind meld to communicate with the creature. He learns that it is a sentient being of a race called the Horta and is in extreme pain, and the Horta learns enough to etch the ambiguous "NO KILL I" into the rock floor. Kirk summons McCoy, though Spock thinks his medical training will be useless on the Horta.
Another mind meld reveals that the Horta is preparing for the extinction of its race. It directs the Humans to find their pump in the "Chamber of the Ages." Kirk tells Spock to communicate to the creature that they are trying to help. He goes to the Chamber and finds about a million silicon spheres, which Kirk and Spock now understand are Horta eggs, ready to hatch.
But several miners, held apart from the confrontation by the Enterprise security team, overpower it and stream into the confrontation. Kirk vows to kill the first man who fires and explains that the Horta is a mother reacting naturally to the destruction of thousands of her children. The Miners are appalled at this terrible misunderstanding and calm down. Spock adds that the Horta "harbor ill will towards no one." Kirk envisages a collaboration between the miners and a new generation of Horta who can tunnel through rock with as much ease as they do walking through air.
The Miners are open to the idea, but Spock mentions a serious caveat: the Horta is seriously wounded and it may die. However, McCoy promptly contradicts that, announcing he has successfully treated the Horata. Dr. McCoy with some pride explains that he troweling thermal concrete, a building material used for emergency shelters that is largely made up of silicon, into the Horta's wound and he expects her to fully recover. With that crisis averted, Kirk has Spock make contact with the Horta to explain the bargain. Spock reassures Kirk that the Horta will likely agree to it with her having a very logical mind; a curiously refreshing change from humans in his opinion.
The crew returns to the Enterprise and Vanderberg radios in that the Horta have started hatching and have already found new mineral deposits. He even mentions that one can get used to their appearance. Spock says the Horta told him the same thing about Humans. He counters some teasing from McCoy by replying that the Horta especially admired Vulcan ears.
- "Captain's log, stardate 3196.1. A distress call from the pergium production station on Janus Six has brought the Enterprise to that long-established colony. Mister Spock, Doctor McCoy, and I have beamed down to meet with Chief Engineer Vanderberg, administrative head of Janus Six."
"When that creature appears, men die."
- - Vanderberg, on the Horta
"Kiss it! Baby it! Flatter it if you have to! But keep it going."
- - Kirk to Scott, on fixing the circulating pump
"Mr. Spock, we seem to have been given a choice: death by asphyxiation or death by radiation poisoning."
- - Kirk
"There's nothing more dangerous than a wounded animal."
- - Kirk to Giotto, after the Horta flees
"Either one of us by himself is expendable. Both of us are not."
- - Kirk, to Spock
- - Spock, sensing the Horta's agony
"No kill I. What is that? A plea for us not to kill it? Or a promise that it won't kill us?"
- - Kirk, reading the Horta's message
"I'm a doctor, not a bricklayer!"
"You're a healer. There's a patient. That's an order."
- - McCoy and Kirk, on treating the Horta
"By golly, Jim! I'm beginning to think I can cure a rainy day!"
- - McCoy, after healing the Horta
"The Horta has a very logical mind. And after close association with Humans, I find that curiously refreshing."
- - Spock, to Kirk
"I suspect you're becoming more and more Human all the time."
"Captain, I see no reason to stand here and be insulted."
- - Kirk and Spock
Story and production
- This episode was originally scheduled to be filmed before "This Side of Paradise", with Ralph Senensky assigned to direct it, but during pre-production the two episodes and the directors were switched because producer Gene L. Coon thought "Devil" would be a tough assignment to first-time Trek director Senensky. 
- Gene Coon's original script featured a different chemical substance as the base of the Horta, but researcher Kellam de Forest corrected it to silicon, as the original choice seemed to be even theoretically impossible. (Inside Star Trek - The Real Story)
- Coon's original script called the planet Thetis Six. Researcher Kellam de Forest pointed out that "The name 'Thetis' has already been assigned by astronomers to the 17th largest asteroid discovered in 1852 in the asteroid belt between Mars and Jupiter," and suggested the name Janus VI instead (de Forest Research Report, December 22, 1966, Gene Roddenberry Papers, UCLA Special Collections, Box 11, Folder 6).
- NBC announced that Star Trek will be renewed for a second season next fall, during the closing credits of this episode on 9 March 1967. (Inside Star Trek: The Real Story, pp 304-305)
- This episode was the first time McCoy used the phrase, "I'm a doctor, not a…" when Kirk asks him to help the Horta, finishing the line as, "I'm a doctor, not a bricklayer!"
- George Takei (Sulu) and Nichelle Nichols (Uhura) do not appear in this episode.
- William Shatner received a call on the set from his mother informing him that his father had died. The crew was ready to shut down production, but he insisted on continuing. During the rest of the day, Shatner took comfort in Leonard Nimoy, and cinematographer Jerry Finnerman, whose father had died on a movie set less than seven years before. (The World of Star Trek)
- Episode stand-in and background actor Eddie Paskey recalled shooting certain scenes in this episode and had high praise for Shatner during this time, stating, "as soon as we wrapped with that show, Bill left. We came to find out that as he shot all day long, he was preparing to get on an airplane to go home – his father just passed away. No one knew until he was actually gone. And, y'know, that says a lot for the dedication of the man."
Props and sets
- Janos Prohaska, the creator of the Horta costume, actually wore it into producer Gene L. Coon's office, as if to say "Look what I designed". Coon said "That's great! What is it?", and Prohaska said "I don't know. It can be whatever you want." Coon replied "I'll write a script around it", and he wrote this episode in four days so the costume could be used. (The World of Star Trek)
- The oversized microbe from the final episode of the 1960s version of The Outer Limits (titled "The Probe", with Peter Mark Richman) was the basis for the Horta. It was also designed and performed by Janos Prohaska.
- The unbroken Horta eggs were toy bouncing balls painted gold.
- The scene of Kirk, Spock, and McCoy discussing the silicon spheres and the murders was originally set in the Enterprise briefing room, but when the death of Shatner's father halted production, it was re-written to be set in Vanderberg's office instead, so that moving to a new set would not slow the filming even more. (These Are the Voyages: TOS Season One)
- A portion of a Horta tunnel was later seen in "Patterns of Force" as the entrance to the Underground's cavern.
- The prop used to depict the damaged reactor for the colony was previously used as the damaged Enterprise transporter circuit found in engineering that Kirk's double phasered in "The Enemy Within".
- The clubs used by some of the Janus VI colonists during their hunt for the Horta appear to be of the same design used by Kirk during his fight with Spock in the transporter room in "This Side of Paradise".
- The only interior portion of the Enterprise seen in this episode is the bridge, and that in only two scenes – when Scott speaks with Kirk about replacing the circulating pump, and at the end, after the landing party has returned to the ship.
- The miners' jumpsuits were worn in a number of subsequent episodes: on various Denevans, including Kirk's brother, Sam, in "Operation -- Annihilate!", on an Argelius II bar patron in "Wolf in the Fold", on Alvin and Robert Johnson in "The Deadly Years", on two Deep Space Station K-7 bar patrons and Lurry in "The Trouble with Tribbles", on Rojan and Tomar in "By Any Other Name", on Professor Starnes and other Triacus colonists in "And the Children Shall Lead", on Linke and Ozaba in "The Empath", on corpses in "The Lights of Zetar", and on Dr. Arthur Coleman in "Turnabout Intruder".
- Although the uniforms come in various colors, Vanderberg is the only miner who wears a yellow specimen, and Ed Appel uniquely wears a purple one, which apparently was later worn by the dead technician on Memory Alpha in the third-season episode "The Lights of Zetar", as well as on Ozaba in "The Empath".
- Although Giotto is identified as a lieutenant commander, he wears a full commander's two solid rank stripes.
- This is the only episode in the original series in which the distinction is drawn between "phaser one" and "phaser two."
- Pergium mining was later referenced in VOY: "Fair Trade" and DS9: "Prodigal Daughter".
- Although having not appeared in this episode, Sulu mentioned "the Hortas of Janus VI" in "That Which Survives".
- This is the only episode of Star Trek: The Original Series that begins without the Enterprise or its crew being involved in the teaser scenes before the main credits, and the only episode that has no female speaking parts.
- Gene Roddenberry was impressed with the way this episode explains the behavior of a Star Trek "monster," citing the installment as "a classic example of doing this right" as well as "one of our most popular episodes." He went on to say, "The Horta suddenly became understandable […] It wasn't just a monster–it was someone. And the audience could put themselves in the place of the Horta… identify… feel! That's what drama is all about. And that's it's importance, too… if you can learn to feel for a Horta, you may also be learning to understand and feel for other Humans of different colors, ways, and beliefs." (The Making of Star Trek, pp. 35 & 36)
- Roddenberry picked it as one of his ten favorite episodes for the franchise's 25th anniversary. (TV Guide August 31, 1991)
- In his book Star Trek Memories, William Shatner identified this as his favorite episode, because his father died during filming and Nimoy's delivery of the mind meld lines made him laugh. Shatner insisted on finishing his dialogue scenes and after he left for the funeral, a stand-in (Eddie Paskey) completed his shots with Spock and the Horta, filmed from behind. When the scenes were later edited together, in several sequences, Shatner has his phaser up and the stand-in does not. However, in the later Star Trek: Fan Collective - Captain's Log DVD set, Shatner said that his favorite episode was TOS: "The City on the Edge of Forever".
- Arthur C. Clarke once remarked, in 1995, that of the Original Series, the only episode he could recall was "The Devil in the Dark", stating that "It impressed me because it presented the idea, unusual in science fiction then and now, that something weird, and even dangerous, need not be malevolent. That is a lesson that many of today's politicians have yet to learn." (Star Trek: Four Generations)
- Leonard Nimoy identified this episode's closing banter between Spock and Kirk as one of his favorite scenes to perform. He noted, "It was a wonderful moment which defined the relationship and defined the whole Spock character's existence and his attitude about himself." (Reflections on Spock, TOS Season 1 DVD special features)
- Anthony Rapp cited this as one of his two favorite Star Trek episodes, the other being "Amok Time", and clarified that what he liked about both episodes was how they depicted Spock. Rapp elaborated, "I love […] what those two episodes tell us about him and about what he's going through as a half-Human half-Vulcan person and the mind meld in 'Devil in the Dark' […] Being able to watch Leonard Nimoy do anything, but especially the work that he did in those two episodes, was really extraordinary." 
- Writing about his relationship to Star Trek and his father, Michael Chabon describes this episode:
- "In "Devil in the Dark," which my father had ranked among his Top Five, the Enterprise came to the rescue of a mining colony on the planet Janus VI, where a terrible monster, the Horta, was preying on pergium miners, picking them off one by one. The episode rises above the banality of a premise as old as Grendel, and some creature effects that are truly risible—even to a ten-year-old in 1973, the homicidal Horta looked like an ambulatory slice of Stouffer’s French-bread pizza—by making an honest effort to imagine nonorganic life and then, in the characteristic turn that gives the "Star Trek" franchise its enduring beauty and power, by insisting that fear and prejudice were no match for curiosity and an open mind, that where there was consciousness there could be communication, and that even a rock, if sentient, had the right to life, liberty, and the pursuit of happiness. It was, in its way, a near-perfect example of what had drawn my father, and me, and fans around the world, to "Star Trek" and its successor shows for more than fifty years."
Novelizations and adaptations
- McCoy specifies that he had the Enterprise beam down about a hundred pounds of thermal concrete. In James Blish's novelization of the episode in Star Trek 4, which was generally based more so on early draft scripts than final drafts, states that McCoy only used ten pounds of concrete.
- Bantam Books published a series of novelizations called "foto-novels," which took photographic stills from actual episodes and arranged word balloons and text over them to create a comic book-formatted story. The ninth installment was an adaptation of this episode. In it, Leslie is depicted as thinking to himself, "That Vulcan would have us killed for his precious science!" after Spock instructs the security detail to capture the Horta.
- The Star Trek: Deep Space Nine novel Devil in the Sky is a sequel of sorts to this episode.
- The 2001 WildStorm Comics comic book "Star Trek: Special" featured a short story of the Borg attacking Janus VI.
- "No Kill I" was the name of a Star Trek-themed punk rock band.
- A cat version of "The Devil in the Dark" was featured in Jenny Parks' 2017 book Star Trek Cats.
- Melodic punk band Pseudo Heroes released a song "Devil in the Dark" based on this episode on their 2005 album Nostalgic Lies....
- "The Devil in the Dark" was the third episode of the remastered version of The Original Series to air. It premiered in syndication on the weekend of 23 September 2006 and most notably featured new effects shots of the pergium production station, the Horta burning through a mine wall, and final fly-by of the Enterprise.
Max Gabl was responsible for creating the new CGI for the mining colony, at the time it was his very first assignment on the episode.  Producer Niel Wray was included in the matte shot as one of the two workmen walking into a tunnel in the lower corner. Wray and another member of the team were shot against a blue screen and placed into the matte. Niel was chosen because he fit into the costume. 
- The next remastered episode to air was "The Naked Time".
- Story outline by Gene L. Coon: 29 November 1966
- Revised story outline: 5 December 1966
- First draft script by Coon: 19 December 1966
- Final draft script: 22 December 1966
- Additional revisions: 16 January 1967
- Filmed: 16 January 1967 – 25 January 1967
- Day 1 – 16 January 1967, Monday – Desilu Stage 10: Int. Janus VI tunnels
- Day 2 – 17 January 1967, Tuesday – Desilu Stage 10: Int. Janus VI tunnels
- Day 3 – 18 January 1967, Wednesday – Desilu Stage 10: Int. Janus VI tunnels
- Day 4 – 19 January 1967, Thursday – Desilu Stage 10: Int. Janus VI tunnels
- Day 5 – 23 January 1967, Monday – Desilu Stage 10: Int. Janus VI tunnels
- Day 6 – 24 January 1967, Tuesday – Desilu Stage 10: Int. Janus VI tunnels, Vanderberg's office
- Day 7 – 25 January 1967, Wednesday – Desilu Stage 10: Int. Vanderberg's office; Desilu Stage 9: Int. Bridge
- Original airdate: 9 March 1967
- Rerun airdate: 15 June 1967
- First UK airdate: 6 September 1969
- Remastered airdate: 23 September 2006
Video and DVD releases
- Original US Betamax release: 1985
- UK VHS release (two-episode tapes, CIC Video): Volume 14, catalog number VHR 2307, release date unknown
- US VHS release: 15 April 1994
- UK re-release (three-episode tapes, CIC Video): Volume 1.9, 30 December 1996
- Original US DVD release (single-disc): Volume 13, 11 July 2000
- As part of the TOS Season 1 DVD collection
- As part of the TOS Season 1 HD DVD collection
- As part of the TOS Season 1 Blu-ray collection
Links and references
- James Doohan as Scott
- Brad Weston as Appel
- Biff Elliot as Schmitter
- George E. Allen as Engineer #1 (possibly Scott's assistant)
- Jon Cavett as a guard
- William Blackburn as Hadley
- Frank da Vinci as Vinci
- Bob Hoy as Sam
- Eddie Paskey as
- Photo double for William Shatner
- Janos Prohaska as the mother Horta
- Ron Veto as Harrison
- Niel Wray as Miner (remastered)
- Unknown actors as
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- "The Devil in the Dark" at StarTrek.com, the official Star Trek website
- "The Devil in the Dark" at Memory Beta, the wiki for licensed Star Trek works
- "The Devil in the Dark" at Wikipedia
- "The Devil in the Dark" at MissionLogPodcast.com, a Roddenberry Star Trek podcast
| Previous episode produced:|
"This Side of Paradise"
| Star Trek: The Original Series|
| Next episode produced:|
"Errand of Mercy"
| Previous episode aired:|
"This Side of Paradise"
| Next episode aired:|
"Errand of Mercy"
| Previous remastered episode aired:|
|TOS Remastered|| Next remastered episode aired:|
"The Naked Time"