Kathryn Hays            Gem
Alan Berman             Lal
Willard Sage            Thann
Jason Wingreen          Dr. Linke
Davis Roberts           Dr. Ozaba
 
A pair of aliens named Lal and Thann (who look like they're related to the Talosians of "The Cage") capture Kirk, Spock and McCoy, and involve them in a series of experiments that include a mute woman named Gem, who has empathic abilities which allow her to absorbs the pain and suffering of others. The aliens are doing so in order to determine whether or not Gem's race is worthy of being saved from a forthcoming super nova, or if a neighboring planet is worthier to survive.

This is Metzcat's least favorite episode... can you tell?