It is a landmark case in Scottish legal history. In 1778, Joseph Knight persuaded the courts he should be free to leave the employment of John Wedderburn. The obstacle was that Wedderburn had bought Knight as a slave in Jamaica and believed he had the right to hold him in perpetual servitude.

Knight lost his initial case in Perth, but based his successful appeal on the argument that Scots law did not support slavery. Whatever the law in Jamaica, he could not legally be enslaved here. The appeal judges agreed.

You would expect a play on this subject to be celebratory. In the era of Black Lives Matter, isn’t it exactly the kind of feelgood story we need? It even has a happy ending.

Omar Austin as Joseph Knight in Enough of Him PIC: Tommy Ga-Ken Wan

But playwright May Sumbwanyambe is not so sure. For him to write a play like that, a simple tale of good versus evil, he would have to paint Knight as the golden hero and Wedderburn as the villain. That would be of no dramatic interest. He wanted something more nuanced.

“I didn’t want to create any good guys that were too good or bad guys that were too bad,” he says. “That’s a trope we’ve seen in all these slavery narratives. We can’t relate to anybody because they are caricatures of human beings.”

Instead, his National Theatre Of Scotland play Enough Of Him brings together four people in an uneasy status game. There is Knight who is enslaved, yes, but also well provided for and educated. In court, he complains about the quality of his silk stockings. There is John Wedderburn who is a slave owner, yes, but also the man who paid for Knight’s wedding, the baptism of his child and for teaching him to read and write.

Then there are the women. As a working-class servant, Knight’s wife Annie is as low down the pecking order as he is. And as the keeper of the house, Lady Wedderburn has reasons of her own to resent Knight’s relationship with her husband.

“The title Enough Of Him has to relate to everybody,” says Sumbwanyambe. “Within this household, Joseph Knight is saying, “Enough of him, because if I can’t get free of this situation, I’m never going to be a father, earn a wage and live a full life.’ At the same time, Margaret Wedderburn is saying, ‘Enough of Joseph. Enough of slavery. Enough of this prejudice inside my house.’”

For the dramatist, the role of John Wedderburn is especially intriguing. If you take the court documents at face value, he genuinely felt he was behaving in a compassionate way. But might he also have been damaged by his time as a young man in Jamaica?

“The trauma of slavery impacts everybody, not just the person who is being enslaved,” says Sumbwanyambe. “After the brutal things they would have been exposed to, how did they then, ten years later, come back to ‘civilised life’ and not be impacted by that? I don’t believe human beings have changed so much that there wouldn’t have been some kind of post trauma. Wedderburn is saying, ‘Enough of the trauma.’”

It is for all these reasons that he has resisted a triumphal ending. “When you do that kind of ending, you are tiptoeing into propaganda,” he says. “Yes, the judge has made this great decision, but it is reckless to say you can go through all that trauma, then a judge goes, ‘You’re the winner,’ and all of a sudden everything’s just good.”

Enough Of Him is just one of a suite of plays Sumbwanyambe has in mind about Scotland’s forgotten history. Glasgow’s Citizens Theatre has commissioned him to write about the city’s race riots of 1919 when a fight between black and white sailors escalated into a street battle. He has been working with Grid Iron on a site-specific piece for a National Trust For Scotland property associated with Robert Wedderburn, son of a white father and black slave mother. Among several more ideas is another one for the National Theatre of Scotland.

“I’m trying to tell the story of Scotland from the point of view of Scotland’s black sons and daughters from 1770 all the way to 1950, 1960,” he says. “The thing that has shocked me is it’s been relatively easy to chart out the stories of black men and their allyship with white working-class women, because they are there in the archives, but with black women, their lives haven’t appeared to be important enough even to record.”

He is careful to force parallels with the world of today, believing the past is a foreign country. But sometimes the connections are too clear to ignore. Looking through the court records of the Knight-Wedderburn case, for example, he came across a painfully familiar argument from those opposing Knight.

“They said, ‘Oh no, if we give this young black man freedom and don’t treat him like property, we’re going to be inundated by all these black men. It was the same argument they used against abolishing slavery. They’re all going to descend on us! The mirrors of that language bleed in from what we’ve heard from the British National Party. These arguments haven’t just sprung out of nowhere. They’ve been with us. I’m fascinated by what was different back then, but at the same time, we’re asking, ‘What is it about these human beings that speaks to us profoundly today?’”

Enough Of Him is at Pitlochry Festival Theatre from 20–29 October and on tour until 19 November.

Enough of Him: new play remembers the slave who won his freedom in a Scottish court