The U.S.S. Enterprise arrives to investigate reports of an unknown monster deep in the mining tunnels of Janus VI. The being is apparently destroying machinery and killing the miners, and has the ability to burrow through solid rock. Spock attempts a Vulcan mind meld with the entity and discovers that it is an intelligent being. A native of the planet, the Horta is normally peaceful and doesn't mind sharing the planet with the miners, but when they broke into the Horta's hatchery and unknowingly destroyed many of her eggs, it attacked to protect its remaining unborn children.

DeForest Kelley (Dr. Leonard H. McCoy)
Ken Lynch (Chief Engineer Vanderberg)
Barry Russo (Lt. Cmdr. Giotto)
James Doohan (Lt. Cmdr. Montgomery Scott)
Brad Weston (Ed Appel)
Biff Elliot (Schmitter)
George E. Allen (Engineer)
Jon Cavett (Guard)

• The Horta is a silicon-based creature. The silicon nodules the miners discovered were its eggs.
  
• This is the first episode McCoy uses what will become his catch phrase, stating "I'm a doctor, not a brick-layer".
  
• The Horta looked like an over-sized plastic vomit.