On a trip to a planet, McCoy goes mad and steps through a time machine. Capt. James Tiberius Kirk and Spock follow him. It is the early 1940's and America has not yet entered the war. Dr. Leonard Bones McCoy has befriended a peace campaigner who according to history if she lived would prevent America from entering the war. Kirk and Spock have to prevent McCoy from saving her life in a car incident.