Stardate 5943.7: The Enterprise arrives in the Beta Niobe system whose star will go nova in three and a half hours. When they find that all inhabitants of the planet Sarpeidon inside that system have vanished, Kirk, Spock and McCoy beam down to what seems to be a library of the planet's history. There they meet Mr. Atoz, who asks them to select a period of interest and shows them a machine called the Atavachron. After viewing a recording, Kirk inadvertently steps through a time portal and finds himself in what is the planet's equivalent to Earth's 17th century. He is being accused of witchcraft, but he receives help from a man who himself came from the future. Kirk finds an exit and returns to the library. Meanwhile Spock and McCoy, who were trying to find the captain, have ended up in Sarpeidon's ice age. They get help from a lonely woman named Zarabeth, who was exiled to that era. Spock somehow reverts to an emotional state of ancient Vulcans, and he falls in love with Zarabeth. Only because of McCoy's insistence they find the way back to the library, just before the planet is destroyed. Zarabeth, however, has to stay behind, because her physiology has been altered by the Atavachron prior to her transfer to that time.

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