• Production Code: 79
• Stardate: 5298.5
• Original Air Date: Jun 3, 1969
• Writer: Arthur Singer
• Director: Herb Wallerstein
• Guest Stars:
  • Sandra Smith (Janice Lester)
  • Harry Landers (Dr. Coleman)
  • Majel Barrett (Nurse Chapel)
  • Barbara Baldavin (Communications Officer)
  • David L. Ross (Lt. Galoway)
  • John Boyer (Guard)

Visiting Dr. Coleman and the ailing Dr. Lester, a colleague of Kirk's from Starfleet Academy who has always envied him due to her inability to achieve a captaincy in a male-captains-only Starfleet, Kirk is rendered unconscious by Lester. It turns out to have been a trap, and Lester puts herself and Kirk into an unknown device that transfers their minds into one another's bodies. Lester, in the form of Kirk, doesn't have time to kill Kirk (now in the female body). Lester and Coleman make every attempt to leave Kirk on the planet, but must bring "her" aboard to save face. Kirk, still suffering a severe shock from the mind transfer, is unable to warn McCoy about Lester's plan to command the Enterprise (especially when Lester keeps ordering Kirk sedated). Lester, however, is unable to conceal her lack of knowledge of command procedures and, more specifically, Kirk's character, and when Spock learns the truth and attempts to help Kirk, Lester has him placed under arrest and tries to speed Spock's court-martial toward a conclusion which would have Kirk and Spock executed.