Set in 18th-century England, “Belle” tells a little known story of a fascinating figure who may have influenced Britain’s highest judge in his decisions regarding slavery.

Dido Elizabeth Belle was the illegitimate mixed-raced daughter of a Royal Navy officer. Details of her life exist, but they amount to little more than sketches.

“Belle,” a British production “inspired” by Belle’s life, fictionalizes and adjusts what’s known about her into a narrative that’s both stilted and melodramatic. The finished film is a second-tier “Amistad” (Steven Spielberg’s 1997 American-set slave-ship drama) meets Jane Austen’s angst-ridden tales of young English women desperate to secure husbands of means. Amidst the movie’s hand-wringing and thwarted romance, the costumes are excellent.

The melodrama starts immediately, when Dido’s father, the world-traveling Sir John Lindsay, asks his uncle, William Murray, and Murray’s wife, Elizabeth Finch, to raise his daughter, a child born to a slave Lindsay met while serving abroad. It so happens that Murray is a man of extraordinary power, the lord chief justice of England and Wales. Belle’s father never again appears in the film.

Belle is raised alongside her cousin, Elizabeth, another great-niece in Murray’s care, at the grand Kenwood House estate in Hempstead. Almost no time is spent on the girl cousins’ childhood. The film cuts to the chase, skipping ahead a decade and more to Belle and her cousin’s young adulthood.

Misan Sagay’s script builds a significant portion of its drama from the contrasting places Belle and her cousin occupy in their great-uncle’s household. While Belle is loved, educated and not raised as the slave, her mixed-race heritage prevents her from being her cousin’s equal.

The latter friction is played out in parlors, parties, offices and over a breakfast table. Murray’s guests at Kenwood House gossip about the young beauty whose mother was a slave.

As for marriage, Belle, according to her great-uncle and aunt’s reasoning, can’t marry. Because of her mixed-race lineage, an English gentleman of noble standing cannot accept her as a wife. And a man of lesser social position would diminish what standing she has.

Gugu Mbatha-Raw stars as Belle, a young woman coming to terms with race and slavery in the 18th century as well as her own challenging place in the world. The script doesn’t do an adequate job of illustrating these awesome tasks.

Mbatha-Raw has passionate scenes, shared with her character’s aunt and uncle, cousin and the viciously racist brother of a young man who courts Belle. But the script’s artifice and the director Amma Asante’s stagey theatricality stop “Belle” from resonating with impact anything close to, for instance, “12 Years a Slave” or “Beasts of the Southern Wild.”

Mbatha-Raw gets fine support, however, from British veterans Tom Wilkinson and Emily Watson as her loving uncle and aunt. She’s in lesser company with the one-dimensional Sam Reid, who co-stars as John Davinier, Belle’s lover and her uncle’s legal apprentice.

Mbatha-Raw’s own lightweight performance and her character’s continuously frustrated romance with Davinier conspire with a storyline that plays like conjecture to make Belle and Davinier’s relationship another false note. The movie never achieves the clarion ring of great drama or groundbreaking history.