It's the late eighteenth century. Dido Elizabeth Belle, the mulatto illegitimate daughter of Royal Navy Captain Sir John Lindsay (Matthew Goode) and a West Indian slave, grew up in privilege with her paternal cousin Elizabeth Murray (Sarah Gadon) in the household of their and Sir John's Uncle Lord William Murray, 1st Earl of Mansfield (Tom Wilkinson) and Aunt Lady Mansfield (Emily Watson), he a Chief Justice. While Dido and Elizabeth have always treated each other as sisters and have considered Lord and Lady Mansfield like their own parents, Dido, although living in comfort, is not afforded all that is given to Elizabeth, due to the color of her skin. As such, Dido is not to come out as Elizabeth is in finding a suitable husband who can raise her status in society. Conversely, Sir John, upon his death, provided financially for Dido, unlike Elizabeth's father, who has made no such financial arrangement for her, making Dido financially secure regardless of what happens with her in terms of matrimony. Regardless, Dido still wants all that it means to come out, she being casually courted by Oliver Ashford (James Norton), against the wishes of his mother and brother, Lady Ashford (Miranda Richardson) and James Ashford (Tom Felton), who in turn is courting Elizabeth. Bigoted Lady Ashford and James do whatever in their power to make Dido's life a living hell, especially if it looks as if she and Oliver plan to marry. Meanwhile, Dido befriends John Davinier (Sam Reid), the son of a local vicar and an aspiring lawyer, he who is researching a case for Lord Mansfield regarding an insurance claim filed by a slave ship's owner for loss of slaves at sea due to illness. Lord Mansfield and John end up having a falling out about one aspect of the case, John wanting the case to lead to abolishing of slavery in England. Through Dido secretly helping John with the case behind Lord Mansfield's back, she and John fall in love. Such a relationship, if it comes to light, having many obstacles including the objections of Lord and Lady Mansfield in favor of Oliver Ashford as a more suitable husband for Dido.