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.