The third and final installment of the Black Guardian trilogy with the White Guardian tapping the TARDIS' energy in order to get a message to the Doctor and directing the TARDIS to the location that is required. But the message is incomplete, and the location appears to be an Edwardian racing yacht. But many things are considerably more than they seem...
'Enlightenment' is a great story that, unfortunately, suffers slightly in the translation from TV to book, because the visual impact of some of the scenes is lessened in the written word. Barbara Clegg, who wrote the screen play, does a great job in translating the story despite the fact that (I believe) she originally wrote it thinking about how it would look presented visually.
Unlike 'Terminus', 'Enlightenment' makes its place in the Black Guardian trilogy an important aspect of the story overall, and is therefore comfortably placed. Unlike some Doctor Who stories, the bad guys operate on a more intellectual level, so that overcoming them cannot rely upon force of strength. Indeed, the Eternals, who feature in this story, are sufficiently outside normal human morality that they can be cast in roles as both allies and enemies. The Doctor, Turlough and Tegan and have roles to play, and Turlough in particular is in a role that no other regular character in the series could be convincingly cast in.
A worthwhile addition to Doctor Who's history.