Miners on Janus VI are being killed by a creature that turns out to be a mother protecting her eggs.
Miners on Janus VI are being killed by a creature that moves through solid rock and leaves victims burned. Kirk and Spock investigate and discover the creature, called the Horta, is a silicon-based being. Spock mind melds with her and learns that the miners have been destroying her eggs.
The episode is one of the best-loved of the series, with a memorable monster built from a simple costume that turns out to be a sympathetic mother. It is also known for McCoy's line about being a doctor and not a bricklayer, which shows the series' wit and tolerance.