Dido Elizabeth Belle (Mbatha-Raw) has all the difficulties of any young, well-to-do woman in 18th-century England. She can't vote, can't drive, can't go to the movies or leave the house without wearing a corset, and she's concerned she may never find a husband. Being half-black only adds to her troubles; it means she can't even eat dinner with her family. The color of her skin means her rank is somewhere below her own family but above the servants.
But her social standing changes when her biological father, a naval officer who left her with his uncle while he sailed around the world, dies and leaves her a fortune. Cue the suitors who feel that riches will outweigh the color of their wife's skin. When her penniless cousin-but-more-of-a-sister Elizabeth (Gadon) is introduced to society, Dido tags along to keep her company, and the two are thrown where not all men are as virtuous as they seem. At the same time, their uncle, William Murray, 1st Earl of Mansfield (Wilkinson) is judging an insurance case where slaves were drowned for the insurance money, and Dido challenges him to see the slaves as human beings rather than property.
Based on a true story, the film is framed by the painting of a portrat of Dido and Elizabeth together, which can still be seen today in Scotland's Scone Palace. Finding out about Dido and the events that shaped the painting and her life would be cheaper and less time-consuming at the Capri, though. Just saying.