The crew of the U.S.S. Enterprise investigate the planet Sarpeidon whose sun is soon to go nova. Upon beaming to the surface, Kirk, Spock, and McCoy discover the inhabitants gone and a library containing the planet's accumulated knowledge, along with a lone librarian, Mr. Atoz, who has been transferring people into the planet's past using a machine called the "atavachron." The atavachron transfers Kirk to a time fraught with superstitions and he is soon arrested for practicing witchcraft. Meanwhile, in an attempt to follow Kirk, McCoy and Spock are transferred to the planet's ice age where they will freeze to death.

James Doohan (Lt. Cmdr. Montgomery Scott)
Mariette Hartley (Zarabeth)
Ian Wolfe (Mr. Atoz)
Kermit Murdock (The Prosecutor)
Ed Bakey (Fop #1)
Anna Karen (Woman)
Al Cavens (Fop #2)
Stan Barrett (The Jailer)
Johnny Haymer (The Constable)