Star Trek: Turnabout Intruder

The Enterprise speeds to help a science party on Camus II after receiving their distress call. Upon arriving at Camus II, the Enterprise's landing party finds all but two of the science party dead. One of the two, Dr. Janice Lester, is a former love of Kirk's from his days at the Starfleet Academy. When Kirk sits with her, Janice uses an alien device to switch minds with Kirk, leaving him in a woman's body while she takes command of the Enterprise.