"The Devil in the Dark" is the twenty-fifth episode of the first season of the American science fiction television series Star Trek. Written by Gene L. Coon and directed by Joseph Pevney, the episode first aired on March 9, 1967. In this episode, the Enterprise is called to investigate deaths at a planetary mining facility. Spock and Kirk go on an away mission to the facility, leading to them facing off against a deadly subterranean creature. This episode marks the first appearance of Doctor McCoy's catchphrase, "I'm a doctor, not a ..." Plot 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. Captain Kirk, Spock, and McCoy meet with the mine supervisor, Chief Engineer Vanderberg. During the briefing, Spock's attention is drawn to a silicon nodule on Vanderberg's desk, which Vanderberg dismisses as a geological oddity. They are alerted to a problem in the colony's nuclear reactor, and find its guard killed and the main circulating pump stolen. The part has long since gone out of production, so no replacements are available. 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. Kirk and his security team search for the creature. Spock, suspecting it may be a silicon based lifeform, modifies their phasers to be more effective against silicon. They encounter the creature—which has the appearance of molten rock—and fire upon it, breaking a piece of it off. The creature flees by burrowing through a rock wall. Spock analyzes the fragment, whose composition resembles fibrous asbestos. He deduces that it burrows through solid rock by secreting the same corrosive substance that has killed the miners. Spock adjusts his tricorder to scan for silicon-based life, and confirms that the creature is the only such lifeform for miles. Kirk and Spock happen upon a chamber containing thousands of the silicon nodules. The creature causes a cave-in that separates Kirk from Spock. Though Spock urges Kirk to kill it, Kirk observes the creature backs off whenever he aims his phaser at it. Spock finds a way around the cave-in and joins Kirk. He attempts a mind meld with the creature, but perceives little but intense pain. The