Dr. 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: Save her, do as your heart tells you to do, and millions will die who did not die before.
Edith Keeler: [to Kirk] I still have a few questions I'd to ask about you two. Oh, and don't give me that "Questions about little old us?" look. You know as well as I do how out of place you two are around here.
Spock: Interesting. Where would you estimate we belong, Miss Keeler?
Edith Keeler: [to Spock] You? At his side, as if you've always been there and always will.
Edith Keeler: [to Kirk] And you... you belong... in another place.
Edith Keeler: [privately with Kirk] Why does Spock call you "Captain"? Were you in the war together?
Capt. Kirk: [warmly but discreetly] We... served together.
Edith Keeler: And you, um, don't want to talk about it? Why? Oh. Did you... did you do something wrong? Are you afraid of something? Whatever it is, let me help.
Capt. Kirk: "Let me help." A hundred years or so from now, I believe, a famous novelist will write a classic using that theme. He'll recommend those three words even over "I love you."
Spock: [to Captain Kirk] I'll finish with the furnace.
Edith Keeler: [to Captain Kirk] "Captain." Even when he doesn't say it, he does.
[Having arrived in Earth's distant past, Captain Kirk steals clothing so he and Spock can blend in but is halted by the sudden appearance of a policeman]
Policeman: [seeing Kirk with an armload of clothing] Well?
Capt. Kirk: You're a police officer. I recognize the traditional accoutrements.
Spock: [in regards to his own appearance] You were saying you'll have no trouble explaining it.
Capt. Kirk: [on the spot to explain Spock's alien features] My friend... is obviously Chinese. I see you've noticed the ears. They're... actually easy to explain...
Spock: Perhaps the unfortunate accident I had as a child...
Capt. 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, uh, skilled, uh, plastic surgeon in civilian life...
Policeman: All right, all right. Drop those bundles and put your hands on that wall there. Come on!
Dr. McCoy: You know, I've convinced myself that this is all in a cordrazine hallucination. But, I've decided you're not.
Capt. Kirk: Spock... I believe... I'm in love with Edith Keeler.
Spock: Jim, Edith Keeler must die.
Guardian of Forever: I AM THE GUARDIAN OF FOREVER.
Capt. Kirk: Are you machine or being?
Guardian of Forever: I AM BOTH AND NEITHER. I AM MY OWN BEGINNING, MY OWN ENDING.
Spock: [archly] I see no reason for answers to be couched in riddles.
Guardian of Forever: I ANSWER AS SIMPLY AS YOUR LEVEL OF UNDERSTANDING MAKES POSSIBLE.
Edith Keeler: If you can leave right away, I can get you 5 hours of work at 22 cents an hour.
[seeing Spock's arcing and sparking tricorder adapter]
Edith Keeler: What... what on earth is that?
Spock: I am endeavoring, ma'am, to construct a mnemonic memory circuit using stone knives and bearskins.
Dr. McCoy: Assassins! Murderers! Murderers! Assassins!
[spots a homeless man]
Dr. McCoy: You! What planet is this?
[the man runs away]
Dr. McCoy: [chasing after him, shouting crazily] No! Don't run! I won't kill you! It's THEY who do the killing! Don't run! I won't kill you!
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 duo-dynetic field core...
Capt. Kirk: [interrupting] Uh, Mr. Spock, I've brought you some assorted vegetables, baloney and a hard roll 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. Mr. Spock, this bag does not 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 is hardly very far ahead of stone knives and bearskins.
Captain Kirk: [having stepped into Earth's past] We seem to be costumed a little out of step with the times.
Spock: I'm afraid I'm going to be difficult to explain in any case, Captain.
Captain Kirk: Well, Mr. Spock, I... if we can't disguise you, we'll find some way of... explaining you.
Spock: That should prove interesting.
Dr. McCoy: [after coming out of a faint] The most common question to ask would be "Where am I?" I don't think I'll ask it.
Edith Keeler: Why not?
Dr. McCoy: The only possible answer would conclusively prove that I'm either unconscious or demented. This looks like old Earth around 1920, '25.
Edith Keeler: Would you care to try for '30?
Dr. McCoy: I AM unconscious or demented.
Edith Keeler: I have a friend that talks about Earth the same way that you do. Would you like to meet him?
Dr. McCoy: I'm a surgeon, not a psychiatrist. I am Leonard McCoy, Senior Medical Officer aboard the U.S.S. Enterprise.
Edith Keeler: I don't mean to disbelieve you, but that's hardly a Navy uniform.
Dr. McCoy: That's quite all right. That's quite all right, dear, because I don't believe in you, either.
Capt. 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.
Capt. Kirk: Sorry.
Capt. Kirk: Time we faced the unpleasant facts.
Spock: First, I believe we have about a week before McCoy arrives, but we can't be certain.
Capt. Kirk: Arrives where? Honolulu, Boise, San Diego? Why not Outer Mongolia, for that matter?
Spock: There is a theory. There could be some logic to the belief that time is fluid, like a river, with currents, eddies, backwash.
Capt. Kirk: And the same currents that swept McCoy to a certain time and place might sweep us there, too.
Spock: Unless that is true, Captain, we have no hope.
Spock: [looking down at his tricorder] Frustrating. Locked in here is the exact 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.
Capt. Kirk: Couldn't you build some form of computer aid here?
Spock: [raising an eyebrow] In this zinc-plated, vacuum-tubed culture?
Capt. Kirk: [applying psychology on Mr. Spock] Yes, well, it would pose an extremely complex problem in logic, Mister Spock. Excuse me. I sometimes expect too much of you.
Capt. Kirk: [narrating] Captain's log, no stardate. For us, time does not exist. McCoy, back somewhere in the past, has effected a change in the course of time. All Earth history has been changed. There is no Starship Enterprise. We have only one chance. We have asked the Guardian to show us Earth's history again: Spock and I will go back into time ourselves and attempt to set right whatever it was that McCoy changed.
Dr. McCoy: Ah! Killers! Assassins! I won't let you! I'll kill you first! I won't let you! You won't get me! Murderers! Killers!
Dr. McCoy: Killers! Killers! I won't let you get me! I'll kill you first! I won't let you get... Assassins! Murderers! Killers!
Spock: Theft, captain?
Captain Kirk: Well, we'll... steal from the rich and give back to the poor... later.
Scott: [Kirk & Spock return from the past through the Guardian] What happened, sir? You only left a moment ago.
Spock: [to Scott, after seeing McCoy return through the Guardian] We were successful.
Guardian of Forever: TIME HAS RESUMED ITS SHAPE. ALL IS AS IT WAS BEFORE. MANY SUCH JOURNEYS ARE POSSIBLE. LET ME BE YOUR GATEWAY.
Lt. Uhura: Captain, the Enterprise is up there. They're asking if we want to beam up.
Capt. Kirk: [softly] Let's get the Hell out of here.