This episode introduces the themes that are explored in the remainder of the series. Clarke expounds on his categorisation of mysteries, self-consciously aping the famous 'close encounters' categorisation used by some UFOlogists: Mysteries of the First Kind, Mysteries of the Second Kind and Mysteries of the Third Kind.

Also in this episode, Clarke views and describes the natural mystery of a solar eclipse

This episode covers alleged sightings of giant reptiles and other cryptids including Mokele-mbembe, the giant anaconda, Ameranthropoides loysi as well as excavations of animals such as the moa and the woolly mammoth.

Clarke includes interviews Roy Mackal and James Powell.