The Terratin Incident

While exploring a burnt-out supernova, the U.S.S. Enterprise responds to an old-style distress signal containing the word "Terratin," and follow it to its source: an uninhabited planet about to shatter from internal stress. However, a mysterious beam strikes the ship and the crew begins to shrink... and shrink... On the planet, Kirk finds a miniature city called Terratin and soon discovers that it is a lost Earth colony that has mutated because of the supernova's radiation.

Synopsis adapted from TVmaze (CC BY-SA 4.0)