In London in 1947, a white London office worker named Ruth Williams (Rosamund Pike) goes to a party with her sister and meets a young black man (David Oyelowo). She soon finds out that he is Prince Seretse Khama from Botswana who is soon to be King, having lost his father. When Seretse proposes marriage to her, Ruth's father disowns her. Nonetheless, they are married, but they have no idea what an outcry their union has caused.
When Seretse takes Ruth back home to Botswana to live, his uncle turns his back on him, stating that Seretse is unfit to rule. The English and South African governments are also against the union, and set up a ruse to get Seretse and Ruth to return to England, where they plan to separate the two.