• Production Code: 71
• Stardate: 5718.3
• Original Air Date: Jan 3, 1969
• Writer: Lee Erwin
• Director: Herb Wallerstein
• Guest Stars:
  • Steve Ihnat (Garth)
  • Yvonne Craig (Marta)
  • Richard Geary (Andorian)
  • Gary Downey (Tellarite)
  • Keye Luke (Cory)

The Enterprise is carrying a new drug to the mental hospital on Elba II, where it is hoped that the last dangerously insane patients in the Federation can finally be treated. But when Kirk and Spock beam down, they do not realize that the facilities have been taken over by the inmates, led by Garth, a former Starfleet captain who has also become a shape-shifter. Before anyone on the Enterprise realizes what is transpiring on Elba II, Garth has activated a shield to prevent the landing party from escaping.