After becoming paranoid as a result of accidently injecting himself with cordrazine, Dr. McCoy beams down to a nearby planet and escapes into a mysterious time portal. The landing party who went after him is then informed that McCoy’s actions in the past have altered the present, and the Enterprise no longer exists. Spock and Kirk enter the time machine to find McCoy, knowing that the slightest action could drastically alter the history of mankind and the fate of millions of people.
“City on the Edge of Forever.” Star Trek. National Broadcasting Company. 6 April 1967.