Executive producer: Gene Roddenberry
Associate producers: Edward K. Milkis and Gregg Peters
Producer Fred Freiberger
Co-producer: Robert H. Justman
Story consultant: Arthur H. Singer

Writer: Jean Lisette Aroeste
Dir.: Marvin Chomsky

Captain Kirk: William Shatner
Science Officer, First Officer, Cmdr. Spock: Leonard Nimoy
Dr. Leonard "Bones" Mc Coy: DeForest Kelly
Lt. Sulu: George Takei
Lieutenant Commander Montgomery Scott: James Doohan
Mr. Atoz: Ian Wolfe
Zarabeth: Mariette Hartley

Stardate 5943.7

A sun is going to explode soon. Orbiting around it is the planet Sarpeidon. The crew needs to make contact with the people to plan for an evacuation. as they suddenly detect an energy field coming from the planet. Kirk, Spock and McCoy beam down and find themselves in a library of a nearly deserted planet. Only Mr. Atoz is left on the planet who wonders why they are so late. He wants to prepare them and asks whether they have chosen a time. As they show their interest in the recent history because the cannot explain for themselves where everybody has gone, he wonders and says that hardly anybody has been interested in the recent history. Using the "atavachron", a machine to bring people into a time before this, he helped everybody to go back in time wherever they wanted to go to. To decide he offered them help in form of discs, archived from "A to Z", on which they could see and hear about the time they had showed interest in.

Kirk looks at one of the discs and suddenly hears a scream. Obviously a Lady is in danger. Speeding through the gate in order to help her Kirk runs to the place he hears the screams coming from. He finds himself in an ancient period of time where a Lady is harassed by several men –seemingly State officials. He intervenes to her help and does not get any thanks. She is a thief and makes fun of his action. Spock and McCoy now also run through the atavachron to assist their captain and arrive at the time they had on their disc: the Ice Age of the planet. Spock and McCoy can still hear the captain and speak to him near the entrance of the gate of time. Their voices and Kirk answering gives the officials the impression that Kirk is involved in witchcraft. He gets arrested and imprisoned.

Shivering in the cold, Spock and McCoy suddenly – and totally unexpected – see a figure standing in the middle of the icy landscape. The figure indicates them to follow. They do. In a warm cave the person reveals herself to them: She is Zarabeth, exiled by a hostile Sarpeidon ruler long ago, she's been condemned to live alone in the ice age. She had been "prepared" and thinks that they were too. From her they now learn that it is not possible for them to ever return. Arrested, Kirk provokes some of the high officials and instinctively notices who reacted to certain words others would not have known what he is talking about. By this he found out who of them was a traveler in time, too. 

He blackmails him into helping him out, but this takes time, for the person does not want to risk his own fate in his newly begun life. In the cave McCoy has become sick. Weakly he lies on his bed and Zarabeth realizes that they have not been prepared. Still, loneliness for so many years has made her desperate for company. Living in a time some thousands of years ago, Spock changes into a Vulcan he would have been thousands of years ago: Thousands of years ago he would have eaten meat, he would have fallen in love with a woman and he would have retaliated McCoy's offenses. He does now all a Vulcan would have done at that time. Zarabeth and Spock grow to love each other. Meanwhile Kirk is shown back to the place where the entrance from the library of the future is to make his way out of this time. Back in the library and with the assistance of Mr. Atoz he finds out that he's an image of a man – appearing in several Mr. Atozes' shapes.

 The love of Spock and Zarabeth has grown. Learning that they have not been prepared, she knows that they will die if they stay. It is too hard for her, but she has got no choice: McCoy is right – they have to return. Now Zarabeth shows them the entrance, Kirk guides them by his voice to find the exact spot and McCoy leaves. Spock knows, he has to leave, too. And he has to leave Zarabeth behind – and they have to part. Still affected by the changes in him having caused him to be a Vulcan he would have been thousands of years ago, Spock lets out his pain and anger about having to leave by showing a fist to the gate. Knowing them to be safe Mr. Atoz now leaps through the gate himself- into the time he has chosen to live in. The crew now knows what has happened to the population of Sarpeidon and the Enterprise leaves before the sun explodes.