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.
I confess that I'm a little surprised to see you. I had thought that everyone had long since gone.
But the surprise is pleasant one. After all, a library serves no purpose unless someone is using it.
Kirk and McCoy examine data storage discs with scenes of an Elizabethan era and an Ice Age, respectively. Meanwhile, Spock talks to Atoz, who is tinkering with a large device that he calls an Atavachron. He explains that it will prepare them once they make their choice. They're interrupted when Kirk hears a scream from a nearby doorway. He runs through it… and vanishes. Spock and McCoy follow through together a few seconds later, only to find themselves in a barren icescape.
Interesting nomenclature. How does it work?
Oh, no, sir, no. I must ask you not to touch the controlling mechanism. Return and make your selection.
When you have chosen, I will prepare you through the Atavachron.
Kirk is in an alleyway in the Elizabethan setting he saw on the data disc moments ago. He sees two fops molesting then woman who screamed and drives them off. She thanks him but he calls out to McCoy and Spock. They've discovered their phasers don't work but can hear Kirk's voice. The woman runs away in superstitious fear, while Spock concludes that the doorway is a time portal and the Sarpedians fled through time to escape the coming nova. The fops bring back the local constable and overhear the conversation, and assume that Kirk is consorting with demons. They knock him out before he can find the time portal on his side.
Lord help us. What's that?
It's spirits. Away. Away, spirit, and let honest men approach.
Unable to find the portal while searching separately, or home in on the captain's voice, Spock and McCoy have no choice but to try and seek shelter. McCoy collapses from the cold and tells Spock to go on. The Vulcan tries to help the doctor without success, and a fur-clad figure appears and guides them to a nearby heated cave. Once Spock makes McCoy comfortable, she introduces herself as Zarabeth, a political prisoner sent through the time portal. She initially doubts their existence but Spock convinces her that they're real.
I am not from the world you know at all. My home is a planet millions of light-years away.
Oh, how wonderful! I've always loved books about such possibilities.
The local Prosecutor arrives to interrogate Kirk. The captain claims to be from an island named Earth but the Prosecutor has no knowledge of it. However, when Kirk mentions the library, the Prosecutor visibly reacts. Kirk tries to follow up but the Prosecutor refuses to talk of it. He suggests he might get Kirk cleared of the charges of witchcraft, while the woman in the next cell claims Kirk cast a spell to make her steal. When Kirk insists on talking about the library, the Prosecutor leaves before he can be incriminated.
I know of no Atoz.
Yes! Yes! Atoz!
I know nothing of this, nothing of these matters!
- I will not hear it!
Let me speak to you!
McCoy briefly recovers conscious and Spock somewhat irritably explains the situation to him. Spock then tries to choose the next logical step but has increasing difficulty. Zarabeth explains that a dictator in her time caught two of her family members conspiring against him, and banished her and her other relatives through the portal. Spock finally asks her to take McCoy through the portal while he searches for Kirk, but she informs him that the Atavachron adapts their bodies to the era they travel to. If they return, they will die in seconds.
I can't go through the portal again. If I do, I will die.
You cannot go back?
None of us can go back. When we come through the portal, we are changed by the Atavachron.
That is its function. Our basic cell structure is adjusted...
to the time we enter. You can't go back.
If you go through the portal again, you will die by the time you reach the other side.
Kirk manages to capture the Jailor when he brings food, and take the keys to make his escape. However, the Prosecutor returns and Kirk pretends he's still imprisoned. He finally gets the Prosecutor to admit he's from the future, and then emerges from the cell and insists the refugee help him find the portal. The Prosecutor warns that anyone prepared by the Atavachron can't return to the present, but Kirk notes that he was never prepared. Realizing Kirk will die in a few hours without preparation, the Prosecutor takes the captain back to the alleyway.
Zarabeth is preparing a meal while McCoy insists they go look for Kirk. Spock notes the doctor is in no condition to go outside and regardless, they have no way back to the present without dying. McCoy snaps at Spock… and Spock snaps back at him. McCoy starts to figure out what's happening to them.
With the Prosecutor's help, Kirk finds the portal and returns to the library in the present. He contacts the Enterprise
but they haven't heard from the others. One of Atoz's replicas tries to send Kirk back through the portal again but he locks it up and then subdues another. The real Atoz intervenes and stuns Kirk unconscious.
To return to the future would mean instant death.
Prepared? I was not prepared. Your Mr. Atoz did not prepare me in any way.
Then you must get back at once! If you were not transformed, you can only survive for a few hours here in the past. Come. Hurry!
Zarabeth finally accepts the presence of Spock and McCoy as real and says she's glad she finally has some company. Given the lack of vegetables, Spock is forced to eat meat and admit that he enjoys it. He then compliments Zarabeth on her beauty and kisses her.
You are beautiful. More beautiful than any dream of beauty I've ever known.
Atoz tries to force Kirk back into the portal but the captain manages to subdue the librarian, then force him to conduct a search of Ice Age data discs to find the one that Spock and McCoy were using. Meanwhile, McCoy finds Spock and Zarabeth together and accuses Zarabeth of lying about whether they can return to the present. Spock attacks him and McCoy says that without the Atavachron's preparation, he's reverting to the barbaric Vulcans of 5,000 years past, their current present. Spock manages to get hold of himself and Zarabeth admits that she doesn't know what will happen to them if they return. McCoy prefers to die finding his own time rather than live trapped in the past, and goes out into the storm to find the portal. Spock and Zarabeth reluctantly follow. Kirk and Atoz find the correct disc and the captain calls out to McCoy. McCoy follows Kirk's voice and Spock tells him to go ahead: he'll stay with Zarabeth. However, McCoy can't pass through the portal by himself. Atoz explains that since McCoy and Spock went through together, they must return together. Spock shares one last embrace with Zarabeth and then goes with McCoy. Atoz slips by them and uses the portal to make his escape, while McCoy considers Spock who notes that Zarabeth is long dead. They beam up to the ship with seconds to spare and leave the system as Beta Niobe goes nova.
There's no further need to observe me, Doctor. As you can see, I've returned to the present in every sense.
But it did happen, Spock.
Yes, it happened. But that was 5,000 years ago. And she is dead now. Dead and buried. Long ago.