1967: City On the Edge Of Forever [http://memory-alpha.wikia.com/wiki/The_City_on_the_Edge_of_Forever_(episode)]. Stardate 3134.0: After taking an accidental overdose of cordrazine, Doctor Leonard McCoy goes back in time and changes history. Joan Collins guest starred in what is one of the most well known of Star Trek stories. She plays Edith Keeler [http://memory-alpha.wikia.com/wiki/Edith_Keeler], a social worker who, in 1930 is knocked down by a car and killed.
McCoy makes her acquaintance and saves her from the accident, unwittingly changing the course of history. Kirk and Spock follow McCoy through the Guardian Of Forever [http://memory-alpha.wikia.com/wiki/Guardian_of_Forever] to stop him from saving Keeler and thus putting the timeline back on track. However, Kirk falls in love with Edith and becomes torn over the fact that he has to let her die to save the future. WATCH!!