When a black African student and a white Englishwoman met at a London Missionary Society dance in June 1947, nobody could have foreseen that the consequences of their encounter would shake the British Empire.

Nonetheless from that night grew an immensely powerful and moving love story, one that is about to become much better-known with the release next month of a major film starring Rosamund Pike and David Oyelowo.

The latter plays Seretse Khama, who came from the British protectorate of Bechuanaland — the country we now know as Botswana.

In the summer of 1947 he was a 26-year-old trainee barrister, tall, broad-shouldered and athletic, who that evening took an instant fancy to slim, strawberry-blonde Ruth Williams, 23. She was from Blackheath, South-East London, and had come to the dance with her sister, Muriel.

Their lower middle-class parents had given their grudging approval to Muriel’s work with the London Missionary Society, on a condition — which seems utterly shocking now — that she didn’t actually bring any black men home. Their other daughter had a surprise in store for them.

Seretse’s feelings for Ruth were fully reciprocated. They kept meeting at dances, talking late into the night, although it was at least another three months before he dared to ask her out on a date. Even that he did with tremendous formality, phoning her in her office, where she worked as a clerk, and asking: ‘Would you do me a great honour tonight?’

The great honour was to accompany him to see The Ink Spots, a popular American singing group. A few months after that, he asked her to marry him. She knew the idea would horrify her father in particular, but she loved Seretse with all her heart, and said yes without hesitation.

They went to celebrate at a Soho restaurant. That evening they kissed for the first time.

For Seretse was heir to the kingship of the Bangwato tribe back in Bechuanaland. And he knew that his choice of wife would be as unpopular there as it was in Britain. His uncle Tshekedi, who had been regent for 23 years while Seretse was groomed for the throne, was implacably opposed to the idea of a white queen, and had a firm ally in the High Commissioner, Sir Evelyn Baring.

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News of the impending marriage soon reached an even more powerful foe. Bechuanaland bordered South Africa, which was during this period implementing its apartheid laws. To the ruling Afrikaner National Party, the very idea of a mixed royal marriage, in a neighbouring country it helped to administer, was an abomination.

Seretse and Ruth could not marry in church. Instead, they quietly tied the knot at a register office, on September 29, 1948.

The newlyweds were not naïve enough to think that their problems were over, but nor did they have any clue of the headaches that awaited them. Three weeks after their marriage, Seretse returned to his homeland alone to prepare the ground for Ruth’s arrival.

At a series of tribal hearings, confronting the fierce disapproval of his uncle and many of his people, he deployed the powers of advocacy he had honed as a trainee lawyer back in London.

Eventually, he persuaded the tribe to accept him as king and Ruth as queen. She joined him, and they set up home in the capital, Serowe. But from the start their happiness at being together was undermined by all those, blacks and whites alike, who wanted them apart.

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One observer wrote that he had seen old Bangwato women ‘fall on one bony knee, clutch at the hem of her flowered frock and kiss it.’ Moreover, just to make her even more precious to them, she was now pregnant.

He did, at least, have the consolation of becoming a global cause celebre, his exile held up as an international symbol of racism.

By now the couple had a son, too, called Ian. They argued over discipline, with Seretse inclined to be more indulgent than Ruth. But the marriage was rock solid. ‘Seretse and I are one race,’ said Ruth. ‘Colour doesn’t enter into it. It never has.’ Their dignity was admirable, their love for each other conspicuous, but the exile endured. 

In 1956, however, the British government decided that Seretse and Ruth should be allowed back. Not out of pity, or recognition that the couple had been treated disgracefully, or even because of continuing pressure from a small group of MPs (led by the young Anthony Wedgwood Benn). No, it was dictated by commercial self-interest.

Geological surveys had revealed large quantities of gold, silver and copper in Bechuanaland, and the Bangwato refused to negotiate mining rights with the British without Seretse guiding them.

She added: ‘I have been so proud of Seretse throughout his exile. Not once has he had the grumps, or taken it out on me for his misfortune.

‘I wonder how many men would have behaved with equal consideration of his wife’s feelings.’

When in due course she joined him, she was greeted rapturously by the Bangwato as ‘Mohumagadi’, meaning Mother of the People.

Ten years later, in 1966, Bechuanaland declared its independence from Britain and became Botswana.

The mineral deposits meant it could stand on its own feet, economically. But the country remained in the Commonwealth, with Seretse — newly knighted by Queen Elizabeth II — as its first president.

He died, aged 59, in 1980. Ruth lived on until 2002, and never dreamt of leaving Botswana. She is buried alongside him, their graves a testament to the power of love, courage and vision, as indeed is the current president of Botswana: Ian Khama, their son.