A former lover of Kirk uses a machine to switch bodies with him and seize command of the ship.
The Enterprise rescues the survivors of an archaeological expedition, and Dr. Janice Lester, a former lover of Kirk, uses an alien machine to switch bodies with him. As Kirk, she takes command of the ship, while the real Kirk is trapped in her body. Spock and McCoy must find out the truth before she has them executed.
As the last episode of the original series, it sees Kirk's old resentments against Janice and her view of the limits placed on women in Starfleet. It remains a controversial story for its handling of those themes, and it closes the three-season run on a dramatic note.