Oliver Ashford (James Norton), the son of a lord, is one of the suitors of Dido (Gugu Mbatha-Raw), center, in the period drama “Belle.” (David Appleby/AP)

Fans of romantic period drama have something to tide them over until the next Jane Austen adaptation. Set in 1769, “Belle” announces its intentions straightaway with a heartfelt reunion between a man and his illegitimate daughter, followed by an exceedingly tearful separation. But even the melodrama can’t put a damper on the remarkable history behind this true story.

Dido Elizabeth Belle was the daughter of British admiral Sir John Lindsay and an enslaved African woman, Maria Belle. After her mother died, and before her father was dispatched to who-knows-where, Dido was placed in the care of her father’s uncle, William Murray. The first Earl of Mansfield, Murray also happened to be lord chief justice, tasked with ruling on cases involving England’s slave trade.

Gugu Mbatha-Raw gives a superb performance as Dido, a very confused young woman who exists in a state of limbo: She is too high-born to mingle with commoners and too dark-skinned to eat dinner with her own family. She is raised with her cousin, Elizabeth (Sarah Gadon), who was abandoned by her father, and the pair grow up like sisters, although Dido isn’t afforded certain basic accommodations that Elizabeth is. And yet, Dido doesn’t question the order of things.

She feels loved by her adoptive parents, a great-aunt and great-uncle played by Tom Wilkinson and Emily Watson, and another aunt (Penelope Wilton), a spinster governess. She also has freedom that Elizabeth does not: Elizabeth has been disowned, left with no dowry, while the death of Dido’s father leaves her a rich woman, so she doesn’t have to marry if she doesn’t want to.

But she has options in the form of two white men willing to buck the system to be with her. One is John Davinier (Sam Reid), a passionate aspiring lawyer and anti-slavery activist. The other, the son of a lord — a more suitable match, according to Dido’s adoptive father — is Oliver Ashford, played by James Norton.

The story transcends the predictable outcome of this love triangle. That thread is supplemented by the recurring theme of liberty and restriction. Dido was freed from slavery and poverty but remains imprisoned by societal prejudice, which pops up in the ugliest ways, especially during an altercation with Oliver’s brother, played by Tom Felton (the erstwhile Draco Malfoy from the “Harry Potter” movies, officially typecast). During one heartbreaking scene, Dido stares in the mirror rubbing desperately at her skin as if her race is merely a smudge that could be wiped away.

Yet Elizabeth, who looks like she belongs among the lords and ladies, is penniless and also shackled by society. Her adoptive parents tell her she has to marry rich, and she laments that she’s become mere property, while Dido can do as she pleases.

The most interesting story line involves Lord Mansfield’s work as he decides the Zong massacre case, in which a ship of slave traders threw 142 slaves overboard, claiming it was necessary because supplies were running low. As if that wasn’t horrifying enough, the owners of the Zong then tried to get insurance money for the financial loss. (The trial was not a murder case but an issue of insurance fraud.) Whether Lord Mansfield will side with the insurance company or the slave traders becomes a point of contention in the family.

The movie packs a lot in, and the quick pace of early scenes can feel like running on a treadmill, but “Belle” settles into a nice rhythm. It ends up having all the requisites of a period drama — a strings-heavy soundtrack, lavish costumes and passionate declarations of love — plus a good deal more.

★ ★ ★

PG. At Landmark’s E Street Cinema, Landmark’s Bethesda Row and Angelika Film Center Mosaic. Contains thematic elements, some strong language and brief smoking images. 104 minutes.