Spock: We'll never reach an Earth base with him aboard. You heard the mathematics of it. In a month, he'll have as much in common with us as we'd have with a ship full of white mice.
Capt. Kirk: I need a recommendation, not vague warnings.
Spock: Recommendation one: there's a planet a few light days away from here, Delta Vega. It has a lithium cracking station; we may be able to adapt some of its power packs to our engines.
Capt. Kirk: And if we can't? We'll be trapped in orbit there. We haven't enough power to blast back out.
Spock: It is the only possible way to get Mitchell off this ship.
Capt. Kirk: If you mean strand Mitchell there, I won't do it! That station is fully automated; there's not a soul on the whole planet. Even the ore ships call only once every 20 years!
Spock: Then you have one other choice: kill Mitchell while you still can.
Capt. Kirk: [offended] Get out of here.
Spock: It is your only other choice, assuming you make it while you still have time!
Capt. Kirk: Will you try, for one moment, to feel? At least act like you got a heart! We're talking about Gary!
Spock: The captain of the Valiant probably felt the same way, and he waited too long to make his decision. I think we've both guessed that.
Capt. Kirk: [drained] Set course for Delta Vega.
[During a game of chess]
Spock: I'll have you checkmated your next move.
Kirk: [chuckles] Have I ever mentioned you play an irritating game of chess, Mr. Spock?
Spock: Irritating? Ah, yes... one of your Earth emotions.
[Kirk checkmates Spock]
Kirk: Certain you don't know what irritation is?
Spock: The fact one of my ancestors married a human female...
Kirk: Terrible, having bad blood like that.
Capt. Kirk: Captain's Log, stardate 1313.8: add to official losses Doctor Elizabeth Dehner - be it noted she gave her life in performance of her duty; Lieutenant Commander Gary Mitchell, same notation.
Capt. Kirk: I want his service record to end that way; he didn't ask for what happened to him.
Spock: I felt for him, too.
Capt. Kirk: [amazed] I believe there's some hope for you after all, Mr. Spock.
Mister Spock: He's reading faster now than just a few moments ago. Is that Gary Mitchell, the one you used to know?
Lt. Mitchell: He's a fool. A fool! He'd seen those points and he hadn't noticed their condition.
Dr. Dehner: How do you know?
Lt. Mitchell: The image of what he'd seen was still in his mind.
Dr. Dehner: Don't you understand? A mutated, superior man could also be a wonderful thing!
Lt. Mitchell: [threatening] Time to pray, Captain. Pray to me!
Lt. Mitchell: Hey, man, I remember you back at the Academy; a stack of books with legs! The only thing I ever heard from an upperclassman was, "Watch out for Lt. Kirk! In his class, you either think, or sink!"
Capt. Kirk: I wasn't THAT bad, was I?
Lt. Mitchell: If I hadn't aimed that little blond technician at you...
Capt. Kirk: You what? You... you PLANNED that?
Lt. Mitchell: Well, you wanted me to think, didn't you? I outlined her whole campaign for her!
Capt. Kirk: I almost MARRIED her!
Lt. Mitchell: You better be good to me. I'm getting even better ideas here.
Kirk: Dr. Dehner feels he isn't that dangerous! What makes you right and a trained psychiatrist wrong?
Spock: Because she feels. I don't. All I know is logic. In my opinion, we'll be lucky if we can repair this ship and get away in time.
[Dr. Elizabeth Dehner breathes her last]
Dr. Dehner: I'm sorry. You can't know what it's like to be almost a god.
Mister Spock: Soon, we'll be not only useless to him, but actually an annoyance.