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 its librarian, Mr. Atoz (), 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 (), who enjoys getting company but tells them return is impossible. Kirk arrives in a Cromwellian period, where he's arrested and suspected of witchcraft.