A mining colony desperately seeks help from the Enterprise in finding and killing a monster that has been tunneling through the mines and killing men. As Kirk, Spock, and a security team track the monster, they learn that it is not inherently hostile but rather misunderstood. Spock mind-melds with it (mind-melding with a rock?), and learns the nature of its existence.
With the intriguing encounter with the Horta, "The Devil in the Dark" represents some of Trek's best ideological values: tolerance for all forms of life, the search for intelligence in unlikely places, and communication with the unknown rather than simply destroying what we fear. And as an action show, the episode works well, too.
Alas, this episode also tends to show some of Trek's unavoidable plotting hokiness—including a painfully obvious "mystery" involving silicon spheres that turn out to be—gasp!—the Horta's eggs! It's perhaps idealistic to a fault, but this episode epitomizes the hopefulness and anti-cynical nature for which Trek is partly known, and for that reason I'm going to give it the benefit of the doubt and then some.