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 is concerned with technology from history that was either ahead of its time and subsequently forgotten, or artefacts which are mysteries in themselves. This includes the Baghdad battery, where German scientist Arne Eggebrecht is shown electroplating a small silver statue with a gold cynanide solution and a replica of the battery using grape juice. There are also segments on the Antikythera Mechanism (including an interview with Derek J. de Solla Price), the Stone Balls of Costa Rica and the so-called 'Skull of Doom' which famously dominates the opening credits of the series.

Clarke opines at the end that had some of these forgotten technologies been developed and not lost that we would have 'colonised the stars' by now.