The Doctor has refused Enlightenment. Turlough is nonchalantly (rather too nonchalantly) picking at his fingernails when the White Guardian offers him a share.
"It's a diamond," he says, staring at the massive, glowing crystal, "The size! It could buy a galaxy. I can have that?"
The White Guardian tells him he can.
"I would point out," interjects the Black Guardian, "that under the terms of our agreement, it is
mine... unless, of course, you wish to surrender something else in
its place. The Doctor is in your debt for his life. Give me the
Doctor, and you can have this," he indicates the crystal, "the TARDIS, whatever you wish."
Turlough is evidently extremely tempted. He has to struggle with himself. When he shoves it towards the Black Guardian, it couldn't be more obvious that he is angry and disappointed with the choice he feels have has to make.
"Here," he shouts petulantly, burying his face, "take it!"
Even so, he does make that choice.
The Black Guardian bursts into flames and vanishes, gurgling and screaming.
"Light destroys the dark," comments the White Guardian. "I think you will find your contract terminated," he tells Turlough.
Turlough takes a smaller crystal from his trouser pocket. It's the one the Black Guardian gave him to seal their bargain. It has turned back, charred by the same flames that consumed the Black Guardian. Turlough drops it.
"I never wanted the agreement in the first place," he mutters.
Tegan is sceptical.
"You believe him because he gave up Enlightenment for your sake," she tells the Doctor.
"You're missing the point," the Doctor says, "Enlightenment was not the diamond. Enlightenment was the choice."
Well, isn't that twee.
Except that it's a bit more grounded that it sounds.
Listen to the words everyone's been using. Diamond. Buy. Terms. Agreement. Contract. Mine. Debt. This is the language of trade, of commerce, of business, of employment, of property, of value and wealth and commodification. Turlough has been desperately trying to wriggle out of a contract, a job. He was basically coerced into it and given a misleading description of what it entailed. He responded by slacking off, dragging his feet, working to rule, fobbing off the boss with endless excuses. The boss got sick of this and tried to sack him. Then, Turlough was presented with an opportunity to take up an altogether more alluring contract. This time, it was a straightforward deal: a dirty great jewel of immense value (Turlough immediately conceives of it in monetary terms) in exchange for a crude hit. No illusions. No lies. Just kill this guy you know to be pretty decent in return for a big pay-off.
He started the story leaning over a chessboard, moving pieces around, deciding which pawn to sacrifice in the cause of winning the game. He ends the story leaning over a table, staring at a prize, given the option to pick it up in exchange for sacrificing one piece. And he can't do it.
This refers to more than just Good vs. Evil. In the cultural landscape of Thatcher's Britain, this is obviously about the morality of being a 'rational actor', a self-interested utility-maximiser, an unsentimental go-getter, etc., etc., ad infinitum, ad nauseum.
But it's also about much older iterations of similar ideas. It's about hierarchy and class and the nightmare of history. The whole story has been a race to the finish, a ruthless competition for a prize. The contestants were bored, amoral exploiters; users of those they call 'Ephemerals', i.e. the short-lived and tiny-brained little people who are only good for labouring to make ships work. The Eternals are always there, fixed and unchangeable, always maximising the utility of the humans that they obtain to work for them (in contracts as coercive and dishonest as the one the Black Guardian foisted on Turlough). They "feed on" them, as the Doctor says. They are hollow and utterly self-involved things, empty without the value they extract from the thoughts and muscles and culture of the 'Ephemerals'. They think of them as worthless, yet obtain everything they have from them. They look like Edwardian officers, ancient Greek warlords, Spanish imperialists... or buccaneering pirates who have entirely embraced the ethic of ruthless, no-holds-barred competition. These are all iterations of the same thing: the top layer, the rulers... to use a more current phrase, the 1%.
They are the 'masters of sail'. Ruling classes from all the way through history, clubbed together and amusing themselves, a band of warring brothers (and sisters), keeping the 'Ephemerals' fooled with doctored grog.
The 'choice' the Doctor mentions is the choice to opt out, at least on a personal level, of that version of history. It's the choice to break that very, very old contract.