Inspirations - Enlightenment

According to its writer, Barbara Clegg, the inspiration for the Eternals came from some well-to-do relatives. Whenever they visited, they expected everyone to keep them entertained - which was very tiring for her and her family.
The Eternals have lived so long that they have totally exhausted their imaginations. They have to turn to the minds of "ephemerals" to find their diversions there. In Enlightenment, that means that they have taken sailors from different historical periods and brain-washed them into believing themselves still to be on Earth in their own times, whereas they have really been put to work on spaceships which mimic their own vessels, flying round the Solar System as part of a race. The planets act as marker buoys.
All of the crew are humans, but the captains and their senior officers are Eternals.
The mind interference isn't foolproof, as we see that the crew of Striker's ship need to be fed regular doses of grog to keep them under control.
God-like aliens in Doctor Who became more common as the programme progressed, but they were fairly rare up to this point. Even the Time Lords had been shown to have the same ailments as humans. Compare with Star Trek, which featured a god-like alien in every other story (or so it felt). Of the six Original Series movies, three had a god-like element, with an alien purporting to be God himself featuring in the fifth of the series. Often these god-like beings played games with the crew of the Enterprise, and often they came across as quite child-like. Marriner, in Enlightenment, comes across as quite naive and innocent, despite his longevity. The Trek episode "The Squire of Gothos" actually ends with the title character's parents turning up and scolding him before grounding him.

The notion of spaceships floating through space using sails has some scientific background. A number of spacecraft designs feature sails of one type or another, designed to capture solar winds rather than our earthly ones. They work on the principle of radiation pressure to drive them.
Craft such as this feature in popular culture as well.
In 1964, Arthur C Clarke published a short story called Sunjammer, which featured a yacht race in space. The previous year, Pierre Boulle's La Planete Des Singes (Planet of the Apes) featured a spacecraft which was powered by solar sails.

Having ships from different time periods allowed for two common story types to feature within the overall narrative. The main setting is The Albatross, Captain Striker's Edwardian vessel. These sequences allowed the production to visit period drama such as The Onedin Line. Captain Wrack and her ship, on the other hand, allowed the story to play with the Pirate genre.
We should also mention the class system dynamics of Striker's ship. The human crew are an underclass, spending most of their time below decks until they are called upon to go on deck to work for the entertainment of their overlords, the Officer-class Eternals. Their imaginations are exploited as much as their physical efforts. The Eternals are never at any risk to themselves - the worst that can happen is even more boredom - whilst the human crews can be harmed, or even killed.

Lastly, the story has the job of completing the Black Guardian Trilogy, as well as Turlough's introductory story arc. The schoolboy was pretty much sidelined in the last story, and here he very quickly starts to oppose the Black Guardian - to the extent that he tries to kill himself by throwing himself overboard rather than face eternity trapped on Striker's ship. By the end of the story he finally decides to reject his unwanted master. To bring the Black Guardian's part to a close, we have the reintroduction of the White Guardian, once again played by Cyril Luckham (first seen in The Ribos Operation). There had been a theory that the two Guardians were one and the same "person" - showing either side of their character at any one time, but here we see both together - though it is stated that neither can ever really be destroyed without destroying the other. Whilst one exists, so will the other.
The cast for this story had to change between the first rehearsals and final studio recording dates due to strike action and over-runs on Terminus. Peter Sallis was supposed to play Striker, but the new dates meant he was no longer available, and Keith Barron stepped into the role. Other parts were also recast. The only way to save this story was to allocate it studio time which should have gone to the series finale - a return for Davros and the Daleks. That story would have to wait until Season 21.
Next time: the Master returns with a scheme which is so unimpressive that it even gets mentioned in the script...