When the planet Sarpeidon is about to be destroyed by its star Beta Niobe becoming a supernova, Kirk, Spock and McCoy beam down and find it evacuated except for Atoz, its librarian, tending with his replicas a collection of unusual discs, which play the planet's history and (to their surprise) allow their users to travel into the past trough the atavachron, a time machine, into the periods each was studying on disc, but they jump in unprepared. McCoy and Spock find themselves locked in a planetary ice age 5,000 years ago, where Spock reverts to the barbaric age of the Vulcans, hence touchy and intensely attracted to the political exilee Zarabeth, who enjoys getting company but tells them return is impossible. Kirk lands in a Cromwellian period, where he's arrested and suspected of witchcraft, but 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 ... Written by
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There were two highly successful sequel books, Yesterday's Son, and Time for Yesterday, both by A.C. Crispin. See more
When the prosecutor is talking to Kirk, the mort (female thief) in the next cell accuses Kirk of being a witch. The constable confirms this, saying that Kirk talked to unseen spirits, one of which he called "Bones". Even though Kirk did address Dr. McCoy as Bones through the unseen time portal, the constable was not present when he did. See more
Long ago... Of course. Long ago!