This is the episode where Dr. McCoy, while not himself mentally due to an hypospray accident, changed earth’s history, when he inadvertently traveled to earths path through an alien portal. When Kirk and Spock travel to the past to try to prevent McCoy from altering history, Kirk falls in love with a young lady (Joan Collins) who will have to die for time to resume as it was prior to McCoy’s interference.

This is an episode that I have watched countless times over the years when I  am in the mood to watch a Star Trek time-travel related episode