Kirk's former love from Starfleet Academy, who used an alien device to exchange bodies with him and seize command of the Enterprise.
Dr. Janice Lester had a relationship with Kirk years earlier and came to hate him, and her own life, for being denied the chance to command a starship. Leading an expedition on Camus II, she lures Kirk to an alien machine that transfers her mind into his body and his into hers.
In Kirk's body she takes command of the Enterprise, but her erratic behaviour arouses the crew's suspicion, and Spock confirms the truth through a mind meld. The transfer fails before she can kill Kirk, and she is left distraught in the care of Dr. Arthur Coleman. She was played by Sandra Smith.