 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 trough 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...