An unknown monster threatens a mining operation.
The pergium miners on planet Janus VI are being killed by an unknown creature that can burrow through stone. Chief mining engineer Vanderberg summons the Enterprise to assist in locating and killing the monster. When the creature steals a vital part of the PXK reactor, which regulates the mining colony's support functions, Spock realizes they are dealing with an intelligent life form. After Captain Kirk wounds the entity, Spock performs a Vulcan mind meld. He learns that the creature, which calls itself a Horta, is killing only to prevent the destruction of its thousands of silicon eggs. Dr. McCoy, using construction materials, heals the wounded Horta. The miners attack with the intention of killing the creature, but when they learn "she" is only a defensive mother and they have been killing her unborn children, they calm down. Vanderberg and his people agree to form a symbiotic relationship with the Horta. The freshly hatched children will dig the pergium—while the miners will sit back and become rich.