Trama

When Kirk, Spock and McCoy investigate the disappearance of a doomed planet's population, they find themselves trapped in different periods of that world's past.

When the planet Sarpeidon is about to be destroyed, by its star Beta Niobe becoming a supernova, Kirk (William Shatner), Spock (Leonard Nimoy) and McCoy (DeForest Kelley) beam down and find it evacuated except for its librarian, Mr. Atoz (Ian Wolfe), tending to his replicas. They are a collection of unusual discs, which play the planet's history and (to their surprise) allow their users to travel into the past through the atavachron, a time machine, into the periods each was studying on disc. The crew travels unprepared, McCoy and Spock find themselves locked in a frozen ice age 5,000 years ago, where Spock reverts to the barbaric age of the Vulcans. He becomes very easily agitated, and is intensely attracted to the political prisoner Zarabeth (Mariette Hartley), who enjoys getting company but tells them return is impossible. Kirk arrives in a Cromwellian period, where he's arrested and suspected of witchcraft. He soon realizes the magistrate must be a time traveler like him, and learns not being prepared at molecular level he can return through the time portal, and in fact must do so and contact Spock and Bones in a matter of hours; they must all return to the present and board the Enterprise before the exploding sun destroys the atavachron.—KGF Vissers

Kirk, Spock and McCoy beam down to the planet Sarpeidon to learn what has happened to its population. The planet, which will be completely destroyed when its sun goes supernova in a few hours time, once had a thriving population but has now been reduced to one person, the librarian Mr. Atoz. It would appear that the planet's entire population has gone back in time to a period of their choice in the planet's history. When the three crewman unknowingly have gone through the time portal, Kirk ends up in a society similar to 17th century Earth while McCoy and Spock end up in a frozen ice age. With Kirk arrested and accused of witchcraft and the others having to survive the harsh Arctic-like climate, they have only a short time to find their way back and return to the Enterprise before the planet explodes. For Mr. Spock, he finds a time from before the existence of Vulcan and he's regressing to an emotional state.—garykmcd

Mere hours before Beta Niobe goes supernova, Kirk, Spock and McCoy beam down to its sole satellite, Sarpeidon, a class M planet, whose large population, incapable of space flight, has, nonetheless, completely vanished. Only one active power source remains - a records archive manned by a lone librarian named Atoz along with several replicas of himself. By accident, Kirk and party run headlong into the planet's secret - a time machine through which the doomed population escaped into its own past. Unfortunately, Kirk gets stuck in its Reformation era, arrested for witchcraft, while Spock and McCoy find rescue from freezing in the planet's ice age through a political prisoner condemned to live alone. Not only have these involuntary travelers just hours to return to the Enterprise before the planet's destruction, but their lack of due process by the library's atavachron will cause their deaths on a cellular level if they don't return. However, Spock, without the proper preparation, has reverted back to the barbaric behavior of his ancestors.—statmanjeff

In Star Trek's penultimate episode, Kirk, Spock and McCoy find themselves transporting down to the planet Sarpeidon to investigate (and possibly help) a planet's evacuation before its sun goes supernova. However, all they find is a single librarian, Mr. Atoz, tending a collection of unusual discs, which play the planet's history and (to their surprise) allow their users to travel into the past. McCoy and Spock find themselves locked in the planet's ice age; Kirk finds himself locked in the planet's colonial age; and they must escape back into the present before the sun explodes, and the library's computer is destroyed. To complicate things, Spock finds himself attracted to a beautiful political prisoner named Zarabeth.—JW Kearse

When the planet Sarpeidon is about to be destroyed, by its star Beta Niobe becoming a supernova (within the next 3.5 hours), Kirk (William Shatner), Spock (Leonard Nimoy) and McCoy (DeForest Kelley) beam down and find it already evacuated except for its librarian, Mr. Atoz (Ian Wolfe), tending to his collection of 20,000 Verism tapes. This seems impossible since the planet and its civilization did not have space flight capabilities. They are a collection of unusual discs, which play the planet's history and (to their surprise) allow their users to travel into the past through the Atavachron, a time machine, into the periods each was studying on disc (Kirk hears a woman scream and steps through the door of the Atavachron. Spock and McCoy follow him but step into the period that McCoy was studying) . Mr Atoz has replicas to help him in the task. He says that all the inhabitants of the planet have safely traveled to wherever they wanted to go, expect the 3 of Kirk, Spock and McCoy. Atoz plans to join his wife and family when the time comes.

The crew travels unprepared, McCoy and Spock find themselves locked in a frozen ice age 5,000 years ago, where Spock reverts to the barbaric age of the Vulcans. He becomes very easily agitated, and is intensely attracted to the political prisoner Zarabeth (Mariette Hartley), who enjoys getting company but tells them return is impossible. Zarabeth was sent to the ice age as punishment by the ruler Zorcon. The Atavachron adjusts the cell structure to the period they enter. So, if they attempt to go back, their cells will age rapidly and die.

Kirk arrives in a Cromwellian period. Kirk and Spock+ MCCoy are initially able to speak to each other (they also figure out that the inhabitants of the planet escaped the nova by travelling to different points of its past), but neither can find a portal gate to return. Soon Kirk is arrested and suspected of witchcraft (he went through the portal in his star-fleet dress) as he was seen and heard talking to voices in the air (which were basically Spock + Bones in their time period) and presumed that Kirk can speak to evil spirits.

Kirk soon realizes the magistrate must be a time traveler like him (he hesitates when Kirk mentions the library). But the magistrate refutes any knowledge of the Atavachron or Mr Atoz and hurries away from Kirk. Kirk escapes his cell by tricking the guards and finds the magistrate. Kirk learns not being prepared at molecular level he can return through the time portal (Mr Atoz warned Kirk, Spock and McCoy not to step into the portal without being prepared, but they did so anyways), and in fact must do so and contact Spock and Bones in a matter of hours (if they were not prepared, they can only survive for a few hrs in the past); they must all return to the present and board the Enterprise before the exploding sun destroys the Atavachron.

Kirk locates the portal with the help of the Magistrate and returns to Sarpeidon. Scott tells him that the sun goes Nova in 17 mins. Meanwhile McCOy is angry that Spock has completely given up by accepting Zarabeth's word that they cant go back. McCoy says that his love for Zarabeth has compelled him to give up his duty. Spock is even prepared to eat meat (he is a vegetarian) to survive in the harsh climate, as that is the only source of nourishment. He plans with Zarabeth to create a greenhouse inside their shelter to grow plants. Spock kisses Zarabeth. Spock attacks McCoy when he says that Zarabeth lied to them to keep Spock in her time period. Zarabeth knew that she couldn't get back, but maybe Spock and McCoy did have a chance. Spock makes it clear that he wants to stay with Zarabeth.

Meanwhile Kirk forces Atoz to help him find Spock and McCoy. Atoz finds the ice age they went to and Kirk guides Spock and McCoy to the portal with his voice. Spock throws McCoy into the portal, but he cant go through alone and they came through the portal together. Spock is forced to return to save McCoy's life. Spock and McCoy return, Scott beams them aboard and warps out, just seconds before the sun goes Nova.