The city on the edge of forever. The Guardian of Forever struck like a hammer with the imagination shown by the writers.
SPOCK: Incredible power. It can’t be a machine as we understand mechanics.
KIRK: Then what is it?
GUARDIAN: (The doughnut pulses bright in time with the words) A question. Since before your sun burned hot in space and before your race was born, I have awaited a question.
KIRK: What are you?
GUARDIAN: I am the Guardian of Forever.
KIRK: Are you machine or being?
GUARDIAN: I am both and neither. I am my own beginning, my own ending.
SPOCK: I see no reason for answers to be couched in riddles.
GUARDIAN: I answer as simply as your level of understanding makes possible.
SPOCK: A time portal, Captain. A gateway to other times and dimensions, if I’m correct.
GUARDIAN: As correct as possible for you. Your science knowledge is obviously primitive.
SPOCK: Really.
KIRK: Annoyed, Spock?
GUARDIAN: Behold. A gateway to your own past, if you wish.
(A camel train passing a pyramid, ancient Rome - images appear inside the doughnut.)
MCCOY: Killers! Killers! I won’t let you get me! I’ll kill you first! I won’t let you get me! Assassins! Murderers! Killers!
(Scott and a guard manage to grab McCoy, and Spock neck-pinches him. An army of lancers charge across the doughnut.)
KIRK: Spock! If that is a doorway back through time, could we somehow take Bones back a day in time, then
SPOCK: Relive the accident. This time be certain that the hypo accident is avoided. Look at the speed with which the centuries are passing, Captain. To step through on precisely the day we wish
KIRK: Guardian. Can you change the speed at which yesterday passes?
GUARDIAN: I was made to offer the past in this manner. I cannot change.
The “Guardian of Forever” (voiced by Bartell LaRue) explains that it is a doorway to any time and place.
Originally written by Harlan Ellison, and not by the regular team, it eventually got worked over by the in-house stable, and Roddenberry himself did the final rewrite. It was hugely over-budget and drug well over schedule.
[Vaguely reminiscent of Ian Anderson's tortured description of how Locomotive Breath came to be.]
Yet, it might be the best Star Trek TOS episode, ever...