Arriving at the moon Sarpiedon, whose mother planet is due to explode in three hours, Kirk, Spock and McCoy find just what the ship's sensors indicated on the surface - no life forms, though an advanced civilization obviously once existed. But they then find a Sarpiedon's librarian, Mr. Atoz, who tells the landing party that all the people of Sarpiedon have already escaped to safety. The library turns out to be a file of "time periods" into which a device Atoz calls the atavachron can propel them. Hearing a woman screaming, but not realizing that she is one the other side of tha atavachron's time portal, Kirk leaps into a time period similar to the 1800s, and Spock and McCoy stumble into an ice age trying to retrieve him. All three must try to survive long enough in their respective environments for the time portal back to Sarpiedon to return - if that moon still exists in the 23rd century for them to return to.
