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 Scotland's Loch Ness Monster and the Ogopogo. At least 50 other lakes in the world are claimed to be likewise inhabited by such giant beasts. Are they fact, hoaxes or merely cases of mistaken identity?