The Enterprise is sent to a mining colony that is being terrorized by a mysterious monster, only to find that the situation is not that simple.
The Enterprise travels to the planet Janus 6 to assist the mining colony there. Kirk, Spock and McCoy beam down to the planet where Chief Engineer Vanderberg tells of a creature loose in the mine tunnels killing some of his men. The monster seems to appear out nowhere then disappears just as quickly. Finding that the creature, known as a Horta, lives in a newly opened part of the underground mining complex, Spock uses the Vulcan mind meld to determine why it is killing the miners.
- Finally a good monster, and no women.
The miners on a subterranean planet leave a man named Schmitter in charge of watching for the monster who has been killing miners.
Schmitter hears noises and he is scared. He screams, and the miners run back. Schmitter has been burned to a crisp, and he is dead. The monster disintegrated him with acid. It is too late to save him, but they send a subspace message to the Enterprise, which is nearby.
Kirk and Spock meet officially with Chief Vandeberg, an old man with white hair, and Ed Appel, a tall angry miner.
Kirk asks for a report from Vanderberg, who tells Kirk that the miners drilled into a new pocket of Pergium, a valuable ore, and suddenly men began dying. Spock is distracted, and holding a bronze globe. He asks "What is this?" Vanderberg says "That is just something we found in the mine. Get down to business."
Meanwhile, Guess What? The monster has crippled the air system by burning it with acid. Now they only have a few hours of air left, while Scotty is trying to make repairs. The Enterprise tries to evacuate all of the miners, but a few, including Ed Appel, refuse to leave, and arm themselves with sticks and clubs as extra security, under the watchful eye of Enterprise Security Chief Giotto, a tall officer who means business.
Kirk and Spock walk follow a tunnel, which splits into two paths. They each take one.
Kirk comes into a mine. He tells Spock on the communicator. Spock says "Captain, stay there." Kirk hears a noise. The wall in front of him is burned with acid. There is the 'Horta', a lava-like mass of ugliness that slithers on the floor, and is as big as four men. Kirk lifts his phaser and points it at the Horta. Earlier, The Horta was shot by a phaser, and it remembers the pain. It backs off.
Every time Kirk raises his phaser, the Horta backs up. Kirk is talking to Spock on the communicator. "It doesn't seem to be harmful, Mr Spock. "I am hurrying to get to you Captain. This creature has killed several miners." Spock arrives and the two of them decide that Spock must use the Vulcan Mind-Meld, and mentally connect with the creature.
Kirk says to Giotto on the communicator "Keep those men there. No one gets by." Kirk also calls the Enterprise and asks McCoy to beam down to heal the wound. McCoy does.
Spock is hearing the thoughts of the Horta. His face is in pain. "The Children. The Children." Spock is in a trance. McCoy is trying to figure out how to heal a creature made of silicon, which is mineral. Kirk goes to the next cavern and sees 1000 silicon globes, like Spock was holding in the office. What the hell?
He comes back in to Spock, McCoy and the Horta. "Spock, what did the Horta tell you?" says Kirk. Spock replies "At first nothing but incredible pain. Your phaser hurt it badly. This Horta is the last of its kind."
"And the silicon nodules?" asks Kirk. Suddenly, the miners with clubs break past Commender Giotto. They are an angry mob now. Kirk holds the phaser up "First men to move gets some of this." Ed Appel says "That monster killed 20 of our men." Kirk yells "And you killed 1000 of her children. There are very few left."
So here is the story. The Horta was the "Mother" of all of these nodules. The nodules are baby Hortas.
Vanderberg says "What happens when these things start hatching?"
Kirk says "Then they will make more tunnels for you, which means more Pergium and more money."
Vanderberg and Appel stop yelling. Kirk says "But the Horta may die." McCoy suddenly shouts "It won't die. I healed it. I believe I can work miracles. I had Scotty beam down concrete and i patched the hole."
Once the crew are back on the Enterprise, Vanderberg calls and says all is well. Do not mess with Mothers. They are fiery when it comes to their children. Wow. All is resolved.