A United Kingdom takes place in the 1940s, and is based on the real story of Prince Seretse Khama, who married a white woman from England and brought her back to Bechuanaland (modern-day Botswana) to presumably rule beside him as queen, once his uncle confirmed him as king. Things don't go as planned, however, and their marriage creates quite a stir, both in Bechuanaland and in England.
Ruth Williams (Rosamund Pike) goes along to a dance with her sister, Muriel (Laura Carmichael), to keep her company. There, she meets Seretse Khama (David Oyelowo), who intrigues her; later, he asks her out on a proper date, and they begin spending more and more time together. At the end of that first date, Khama reveals to her that he's the prince of Bechuanaland, an African country, and that his education in England has ended; therefore, his uncle expects him back in Bechuanaland soon.
Ruth and Seretse know that they probably wouldn't be allowed to marry in Africa, so they marry in England before they leave, and Ruth accompanies Seretse back to Bechuanaland. There, they meet stern opposition, both by his uncle, the current regent of the country, and by England, which at the time was basically 'moderating' Bechuanaland, as it was a protectorate of the English government.