• Production Code: 68
• Stardate: 5710.5
• Original Air Date: Nov 29, 1968
• Writer: Lee Cronin
• Director: Jud Taylor
• Guest Stars:
  • Kathie Browne (Deela)
  • Jason Evers (Rael)
  • Majel Barrett (Nurse Chapel)
  • Erik Holland (Ekor)
  • Geoffrey Binney (Compton)

When a landing party investigating Scalos begins to vanish one by one, Kirk, Spock and McCoy try to find out what is happening before more of the crew disappears, until Kirk himself is abducted. Kirk finds the cause to be a group of endangered Scalosians who move faster than human sight or hearing can detect. They need to repopulate their species, and find that speeding human males up to Scalosian speed will meet their needs. Kirk must find a way to get a message to Spock and McCoy, who are working on a cure for the mystery "ailment," as well as stirring up fighting among the Scalosians, before they have control of the Enterprise.