A communication probe arrives in Dr. Huer's office, and delivers a strange riddle: "A man out of old Earth's past, has the key to the next and the last; but you won't solve this riddle, till the end is the middle, and Terran sands disappear with a blast." The future survival of Earth depends on whether or not mind probes used on Buck can solve the riddle. Searching for a solution, Buck recounts his previous adventures since his arrival on 25th Century Earth.


Although transmitted afterwards, this episode features linking scenes which are set about a week before the events in the previous episode.