Well, there's not much to be said about this episode that hasn't been said already. It's considered by many to be the best episode of TOS ever. IN 1968 all five of the nominees for the Hugo (a sci-fi award that doesn't apply only to a specific media) for Best Dramatic Presentation were Star Trek episodes, and this was the one that topped the list. That award wouldn't get won by a television show again for another 25 years, when it was taken by the Next Generation episode, "The Inner Light."
What the hell was wrong with Sci Fi on television between TOS and TNG? Why weren't we doing anything?!
Fans of Transformers who never watched Star Trek will find this episode title familiar, as the title of, Forever is a Long Time Coming was something of an homage to this title. As to the city referenced in the title, it's both the ruined city that the Guardian can be found in, and New York City of the past - a city on the verge of two destinies.
Seriously, though. If you only ever watch one episode of Star Trek, this one should probably be it.
Anyway. On to the actual summary.
While the Enterprise circles and studies a planet emitting bizarre temporal waves, an accident with a hypo turns McCoy, temporarily, into a paranoid, ranting lunatic. So, yes, very much like Red Alert in the episode Auto Berserk. He manages to beam himself down to the planet at the center of the temporal waves, and an away party is sent after him. While looking for McCoy, they find an ancient archway, and while investigating it, it begins to speak to them. It calls itself the Guardian of Forever, and speaks in vagaries and riddles that I'll probably end up quoting later. It mentions that it's waited for countless eons to be asked a question... and then it never gives a straight answer. I guess when you're an older-than-time artifact, after awhile dicking people around becomes something of a diversion. A way to, hah-hah pass the time.
The crew determine that the archway is a gateway into other times, and Spock begins to record information on what goes on beyond. However, images flicker through at an incredible rate, and while the tricorder can record it, it's too much for the humans (and Vulcans) to follow. While they're marveling over the discovery, Dr. McCoy bursts past and dives into the portal. Moments later, Uhura announces that they've lost contact with the Enterprise, and the Guardian explains that all that they knew was no more. Being on that planet protected them from the effects, but the rest of history has been altered. There's no more Federation, no more Star Fleet, no more Enterprise.
They get the Guardian to rewind to the same general moment, but due to the speed that time flickers past, they can't be exact. Then Kirk and Spock go to the past at a point before McCoy's arrival, but they don't know how long before.
They arrive during the Great Depression. While in the past, Kirk and Spock meet an extraordinary woman, Edith Keeler, who dreams of a future like the one they're from, who spends her time giving and helping the poor and hungry while encouraging them to live, to survive, because the future is worth living for. She talks of harnessing the atom, of flying to safe, on conquering disease and bringing peace. And she's beautiful. Naturally, Kirk falls head over heels for her. Spock, meanwhile, works to get his tricorder working so he can figure out what caused the divergence.
Eventually, Spock figures it out what happened: in the original timeline, Edith Keeler dies. Very soon. In the altered timeline, Edith Keeler lives, her message of peace grows and spreads and becomes widely adopted throughout the United States... and so the more pacifistic America delays its entry into World War II, and eventually the planet falls to the Nazis. One woman preaching peace is fated, if she lives, to wipe out the very future she dreams of, and Kirk, to prevent that, must stop McCoy from saving the woman he loves.
Meanwhile, McCoy does finally arrive in the past, and also meets Keeler. She nurses him back to health, doing it as part of her usual good works and not mentioning it to her growing love interest. She does remark on his strangeness, though, and comments how "a friend" occasionally talks like him. Later, with Kirk, she lets slip McCoy's name, and Kirk realizes what happened. He orders Keeler to stay put on one side of a busy New York street and runs back to get Spock. Spock emerges from the building where they're renting, and Kirk explains to him that McCoy is there. Then, before they get further, McCoy himself emerges from the building, and the three greet each other joyfully. Keeler smiles to see the three friends' good spirits, and she starts to cross the street to meet them.
She never sees the truck heading for her.
Kirk starts to run to save her. Spock calls for Jim to stop, reminds him, and he freezes. McCoy tries to rush past Kirk to save her instead, and Kirk grabs him. Kirk clings to McCoy, preventing him from rushing out, even as he twists away, unable to watch as she's run down.
They return to the future. Time is restored. The Guardian offers unlimited knowledge in the form of the time recordings, even offers other trips into the past. Kirk turns away.
(Note: No, the Federation isn't quite that stupid. They do revisit the Guardian as a source of knowledge in the animated series, and it's popular fan-theory that the Guardian is the reason why records of the late 20th/early 21st century are sketchy in the TOS time-frame, but seem more detailed and complete come TNG.)
MCCOY: Some heart flutter. Better risk a few drops of cordrazine.
KIRK: Tricky stuff. Are you sure you want to risk
(The hypo is administered and Sulu opens his eyes.)
MCCOY: You were about to make a medical comment, Jim?
KIRK: Who, me, Doctor?
MCCOY: (screams) Killers! Assassins! I won't let you! I'll kill you first! I won't let you! You won't get me! Murderers! Killers!
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.
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.
KIRK: Where is he?
GUARDIAN: He has passed into... what was.
UHURA: Captain, I've lost contact with the ship. I was talking to them. Suddenly, it went dead. No static, just nothing.
KIRK: Kirk to Enterprise. Scotty.
SCOTT: Nothing wrong with the communicator, sir.
GUARDIAN: Your vessel, your beginning, all that you knew is gone.
KIRK: McCoy has somehow changed history.
SCOTT: You mean we're stranded down here?
SPOCK: With no past, no future.
UHURA: Captain, I'm frightened.
KIRK: Earth's not there. At least, not the Earth we know. We're totally alone.
(They head into an alley with washing hanging out on the fire escape.)
SPOCK: Theft, Captain?
KIRK: Well, we'll steal from the rich and give back to the poor later. I think I'm going to like this century. Simple, easier to manage. We're not going to have any difficulty explaining
(Then he sees the policeman.)
POLICEMAN: Well?
KIRK: You're a police officer. I recognize the traditional accoutrements.
SPOCK: You were saying you'll have no trouble explaining it.
KIRK: My friend is obviously Chinese. I see you've noticed the ears. They're actually easy to explain.
(A crowd is gathering.)
SPOCK: Perhaps the unfortunate accident I had as a child.
KIRK: The unfortunate accident he had as a child. He caught his head in a mechanical rice picker. But fortunately, there was an American missionary living close by who was actually a skilled plastic surgeon in civilian life.
POLICEMAN: All right, all right. Drop those bundles and put your hands on that wall there! Come on!
KIRK: You were actually enjoying my predicament back there. At times, you seem quite human.
SPOCK: Captain, I hardly believe that insults are within your prerogative as my commanding officer.
KIRK: Sorry.
SPOCK: Frustrating. Locked in here is the place and moment of his arrival, even the images of what he did. If only I could tie this tricorder in with the ship's computers for just a few moments.
KIRK: Couldn't you build some form of computer aid here?
SPOCK: In this zinc-plated vacuum-tubed culture?
KIRK: Yes, well, it would pose an extremely complex problem in logic, Mister Spock. Excuse me. I sometimes expect too much of you.
EDITH: Well, I could do with some help around here. Doing dishes, sweeping, general cleaning.
SPOCK: At what rate of payment? I need radio tubes and so forth. My hobby.
KIRK: Radio tubes and so on. I approve of hobbies, Mister Spock.
EDITH: Now, let's start by getting one thing straight. I'm not a do-gooder. If you're a bum, if you can't break off of the booze or whatever it is that makes you a bad risk, then get out. Now I don't pretend to tell you how to find happiness and love when every day is just a struggle to survive, but I do insist that you do survive because the days and the years ahead are worth living for. One day soon man is going to be able to harness incredible energies, maybe even the atom. Energies that could ultimately hurl us to other worlds in some sort of spaceship. And the men that reach out into space will be able to find ways to feed the hungry millions of the world and to cure their diseases. They will be able to find a way to give each man hope and a common future, and those are the days worth living for. Our deserts will bloom. (She continues under the dialogue.)
KIRK: Development of atomic power is years away, and space flight years after that.
SPOCK: Speculation. Gifted insight.
KIRK: I find her most uncommon, Mister Spock.
EDITH: Prepare for tomorrow. Get ready. Don't give up.
EDITH: Do you have a flop for the night?
KIRK: A what?
EDITH: You really are new at this, aren't you? A flop is a place to sleep.
KIRK: Oh.
KIRK: We have a flop.
SPOCK: We have a what, Captain?
KIRK: A place to sleep.
SPOCK: One might have said so in the first place.
SPOCK: Captain, I must have some platinum. A small block would be sufficient, five or six pounds. By passing certain circuits through there to be used as a duodynetic field core.
KIRK: Mister Spock, I've brought you some assorted vegetables, baloney in a hard rolls for myself, and I've spent the other nine tenths of our combined salaries for the last three days on filling this order for you. Mister Spock, this bag doesn't contain platinum, silver or gold, nor is it likely to in the near future.
SPOCK: Captain, you're asking me to work with equipment which hardly very far ahead of stone knives and bearskins.
KIRK: McCoy will be along in a few days, perhaps sooner. There's no guarantee that these currents in time will bring us together. This has to work.
SPOCK: Captain. Captain, in three weeks at this rate, possibly a month, I might reach the first mnemonic memory circuits.
(There's a knock at the door.)
KIRK: Your hat.
EDITH: If you can leave immediately, I can get you five hours work at twenty two cents an hour. What? What on Earth is that?
SPOCK: I am endeavoring, ma'am, to construct a mnemonic memory circuit using stone knives and bearskins.
KIRK: How are the stone knives and bearskins?
SPOCK: I may have found our focal point in time.
SPOCK: This is how history went after McCoy changed it. Here, in the late 1930s. A growing pacifist movement whose influence delayed the United States' entry into the Second World War. While peace negotiations dragged on, Germany had time to complete its heavy-water experiments.
KIRK: Germany. Fascism. Hitler. They won the Second World War.
SPOCK: Because all this lets them develop the A-bomb first. There's no mistake, Captain. Let me run it again. Edith Keeler. Founder of the peace movement.
KIRK: But she was right. Peace was the way.
SPOCK: She was right, but at the wrong time. With the A-bomb, and with their V2 rockets to carry them, Germany captured the world.
MCCOY: The most common question to ask would be, where am I? I don't think I'll ask it.
EDITH: Why not?
MCCOY: The only possible answer would conclusively prove that I'm either unconscious or demented. This looks like old Earth around 1920 or 25.
EDITH: Would you care to try for 30?
MCCOY: I am unconscious, or demented.
EDITH: I have a friend that talks about Earth the same way that you do. Would you like to meet him?
MCCOY: I'm a surgeon, not a psychiatrist. I am Leonard McCoy, Senior Medical Officer aboard the USS Enterprise.
EDITH: I don't mean to disbelieve you, but that's hardly a Navy uniform.
MCCOY: It's quite all right. It's quite all right dear, because I don't believe in you, either.
SPOCK: We're not that sure of our facts. Who's to say when the exact time will come? Save her, do as your heart tells you to do, and millions will die who did not die before.
MCCOY: You deliberately stopped me, Jim. I could have saved her. Do you know what you just did?
SPOCK: He knows, Doctor. He knows.
SPOCK: We were successful.
GUARDIAN: Time has resumed its shape. All is as it was before. Many such journeys are possible. Let me be your gateway.
UHURA: Captain, the Enterprise is up there. They're asking if we want to beam up.
KIRK: Let's get the hell out of here.
Transcript.