A mining colony reports a number of mysterious deaths just after they successfully dig to a lower level of a planetoid believed to be uninhabited. The Enterprise arrives, and Kirk, Spock and security officers from the ship begin a hunt for whoever or whatever is responsible for the growing body count. An amorphous creature capable of burning through the indigenous rock is found to be the cause of the deaths as well as a very well-thought out sabotage of the miners' life support systems. Through a mind-meld, Spock communicates with the being - known as the Horta - and finds that it is the last of its kind, a mother laying eggs in the tunnels and caves it builds for itself. But the humans have been discovering and destroying the eggs, and if the Horta cannot bring herself to negotiate with the miners, one party or the other faces extinction.
