The Eye of the Beholder

Beaming down to a planet to search for the crew of the missing ship Ariel, the U.S.S. Enterprise crew is captured by a race of super-intelligent slug creatures and become part of a zoo collection. They find the missing crew also trapped as prisoners in the alien zoo. Now they must somehow establish communications with their evolved slug-like captors who think of them as primitive animals.

Synopsis adapted from TVmaze (CC BY-SA 4.0)