 In the series premiere (this was the first episode of Star Trek aired but not filmed), the Enterprise visits planet M-113 where scientist Dr Crater and his wife Nancy, an old girl-friend of Dr McCoy, are studying the remains of an ancient civilization. The Enterprise is charged with resupplying the couple, and Dr McCoy must give them medical exams. Shortly after their arrival, however, crewmen start turning up dead, drained of salt, and left with unusual red marks on their faces. Someone is not who they appear to be, and Kirk and Spock must unravel the clues to discover why, how, and who is responsible.