enters the Beta Niobe star system. The sun is about to go nova, and the civilization on its one inhabited planet, Sarpeidon, has disappeared without a trace. Kirk, Spock, and McCoy beam down to a building with the only active power source and discover it's a library with data storage discs. An elderly man, Mr. Atoz, comes in and introduces himself as the librarian. He mistakes them for natives and says that they've just barely come in time. He offers them the resources of the library to make their "choice," assuming they know what he's talking about. They leave him behind and walk toward some shelves… and he appears again in front of them. When they ask him about recent history, he directs them to the reference deck, and yet another Atoz. This one says that he is the real Atoz and the others are replicas, and that the landing party should join the rest of Sarpeidon's population and make their escape so that he can join his wife and children and avoid the upcoming nova...Read the full recap
James Doohan (Scotty) does not appear on screen but has several voice-over lines.
The interior of the Enterprise is not seen during this episode. The ship itself is only seen briefly in the first and last scenes.
Mr. Atoz is aptly named: his name, broken down, is "A to Z" which seems quite fitting for a librarian, even considering that his library does not contain books.
The Atavachron is a recycled prop of Gary Seven's computer from "Assignment: Earth."
Mr. Atoz: You're very late!
Kirk: You're a very agile man, Mr. Atoz! Just how many of you are there?
Spock: We're in a wilderness of arctic characteristics!
McCoy: He means it's cold!
Zarabeth: But your friend-- he is ill.
Spock: That is true. If I leave him, he may never regain the ship. He would be marooned in this time period. But he is no longer in danger of death, so my primary duty to him has been discharged. And if I remain here, no one of our party would be able to aid Captain Kirk.
Zarabeth: Oh, you make it sound like an equation.
Spock: It should be an equation! I should be able to resolve this problem logically.
McCoy: Zarabeth, you are a beautiful cook. Have you ever been told that?
Zarabeth: Not recently.
McCoy: Oh, well, you'll find Spock is quite delinquent in those matters.
Zarabeth: I hadn't noticed it.
Spock: Dr. McCoy is making excellent progress.
McCoy: Mr. Spock has been practicing medicine without a license. Don't let him doctor you. I'm the doctor around here.
Spock: And known as the worst patient in the entire crew of the Enterprise.
McCoy: You listen to me, you pointed-eared Vulcan.
Spock: I don't like that. I don't think I ever did, and now I'm sure.
McCoy: What's happening to you, Spock?
Spock: Nothing that shouldn't have happened long ago.
Zarabeth: Do you know what it's like alone, really alone? Weapons, shelter, food... everything I needed to live... except companionship... to send me here alone... if that is not death, what is?
McCoy: Are you trying to kill me, Spock? Is that what you really want? Think. What are you feeling? Rage? Jealousy? Have you ever had those feelings before?
Spock: This is impossible. Impossible. I am a Vulcan.
McCoy: The Vulcan you knew won't exist for another 5,000 years. Think, man. What's happening on your planet right now, at this very moment?
Spock: My ancestors are barbarians, warlike barbarians. Who nearly killed themselves off with their own passions.
McCoy: Spock, you're reverting into your ancestors... 5,000 years before you were born!
Spock: I've lost myself. I do not know who I am.
Spock: There is no further need to observe me, Doctor. As you can see I have returned to the present in every sense.
McCoy: But it did happen, Spock.
Spock: Yes. But that was 5,000 years ago, and she is dead and buried.
Spock says that Vulcan is "millions of light years away." The known galaxy is "only" 100,000 light years away, and later in Voyager, Voyager is thrown 75,000 light years away and takes 75 years to return to the Federation at maximum speed.
DeForest Kelley's lips do not move when he says "You've been dishonest with me, Spock."