The Horta is a globular mass that turns out to be thinking, feeling being. If, indeed, a crew such as those of the enterprise were to exist, they would need to be sensitive to the complexities of various life forms. A miner has died a horrible death, bathed in acid. The other miners, searching for some really valuable ore, have encountered this creature who seems to be able to meld itself with rock. The minders seek assistance and Spock, McCoy, and Kirk come to the rescue. There is the obligatory confrontation with phaser fire and injury. However, things get much more complicated as the actions of the Horta seem to be stimulus/response. The creature seems to understand. Spock uses the Vulcan mind meld to find out what is going on. The creature is badly hurt by the phaser fire and McCoy is asked to heal, leading to one of the best lines, "Dammit, Jim, I'm a doctor, not a bricklayer." The creature is silicon based, hence the reference to repairing rock. It's a sort of touching episode in some ways and has a satisfactory lesson.