Enterprise arrives at the pergium mining colony on planet Janus VI to help the colony deal with a creature that has killed 50 miners and destroyed equipment with a strong corrosive substance. Kirk, Spock, and McCoy meet with the mine supervisor, Chief Engineer Vanderberg and an engineer, Appel, who describe the amorphous creature and its behavior. During the briefing, Spock's attention is drawn to a spherical object on Vanderberg's desk, which Vanderberg dismisses as a worthless silicon nodule, saying that there are thousands of them in the mines, and that they are a mere geological oddity. Suddenly, they are alerted to a problem in the colony's main nuclear reactor, and find its guard killed and the main circulation pump stolen. Chief Engineer Scott rigs a temporary replacement that will prevent critical failure, but a more permanent solution must be found in 48 hours.
