• Production Code: 63
• Stardate: 5121.0
• Original Air Date: Dec 6 68
• Writer: Joyce Muskat
• Director: John Erman
• Guest Stars:
  • Kathryn Hays (Gem)
  • Alan Bergmann (Lal)
  • Davis Roberts (Dr. Ozaba)
  • Jason Wingreen (Dr. Linke)
  • Willard Sage (Thann)

Kirk, Spock and McCoy search for two missing scientists on a planet whose sun is about to explode, but they only find visual logs that show the scientists disappearing. Then the landing party disappears as well, finding themselves trapped by two aliens who snatched the scientists away and experimented on them until they died. The aliens now have Kirk, Spock, McCoy, and a mute empath nicknamed "Gem" by the landing party in their custody, and the captors seem to be intent on repeating the same deadly experiments on their new specimens.