"The Devil in the Dark" is the twenty-fifth episode of the first season of the American science fiction television series Star Trek.

This episode marks the first appearance of Doctor McCoy's catchphrase, "I'm a doctor, not a ..." The USS Enterprise arrives at the pergium mining colony on planet Janus VI to help the colony deal with an unknown creature that has killed 50 miners and engineers, and destroyed equipment with a strong corrosive substance.

They are alerted to a problem in the colony's nuclear reactor, and find its guard killed and the main circulating pump stolen.

Chief Engineer Scott jury rigs a substitute, but it fails shortly thereafter, necessitating the missing part be found and reinstalled before the reactor goes super-critical in 10 hours or is manually shut down, which will suffocate the miners.

The creature, having gained some knowledge of human language from the meld, etches the ambiguous message "NO KILL I" into a rock.

He learns that the creature is called a Horta, and that its species dies out completely every 50,000 years, save for one individual that remains alive to protect the eggs, which are the silicon nodules.

Kirk, Spock, and McCoy return to the Enterprise, prepare to leave orbit, and learn from Vanderberg that the eggs have hatched and already the new Horta have uncovered rich veins of pergium and other valuable metals.

Episode writer Gene Coon was convinced of the costume's effectiveness after an impromptu demonstration by Prohaska in the studios.

His father died during its filming, but Shatner insisted on going through with production, and felt closer to the cast and crew for helping him through the difficult time.

His daughter Eileen continued in his footsteps and served as the president of the Motion Picture Sound Editors, M.P.S.

[citation needed] William Shatner wrote in his memoirs that "The Devil in the Dark" was his favorite original Star Trek episode.

[3] In the documentary 50 Years of Star Trek, Leonard Nimoy also named "The Devil in the Dark" as an "interesting episode", stating "I thought [it] was a wonderful episode about the fear of the unknown, how we fear and even hate something that we don't know anything about, learn who your enemy is, and it's not, maybe then it's no longer your enemy.

Club gave the episode an 'A' rating, describing it as a classic and noting the well-written roles of Kirk, Spock and McCoy.