Its “Masterpiece Theatre” gentility may feel overly decorous after the bracing experience of “12 Years a Slave,” but Amma Asante’s “Belle,” about the beginnings of the abolitionist movement in England, is nevertheless an engrossing, beautifully appointed, if somewhat prosaic and historically loose, docu-drama.

The title refers to the mulatto Dido Elizabeth Belle (played by Lauren Julien-Box as a young girl and then for most of the film by Gugu Mbatha-Raw), the daughter of English gentleman John Lindsay (Matthew Goode) and an African woman, probably a slave. When her mother dies, she’s taken up by Lindsay, who genuinely loves her but must shortly go off to sea under royal commission. He therefore leaves the girl in the care of his uncle, Lord Mansfield (Tom Wilkinson) the Lord Chief Justice of the King’s Bench, and his wife (Emily Watson), who are already guardians of another grand-niece, Elizabeth Murray (Cara Jenkins as a child, and later Sarah Gadon), who embraces her as a sister.

Misan Sagay’s screenplay employs the little that’s know with certitude of Belle—most notably a painting that depicted her and Elizabeth together in an affectionate pose—to construct a tale that connects her presence in Lord Mansfield’s household at his Kenwood estate with his important verdict in the so-called Zong Massacre case of 1783. In that decision Mansfield held for insurers who refused to pay compensation for more than a hundred slaves who were summarily tossed overboard during the voyage to Jamaica, finding that fraud was involved in the crew’s decision to jettison this portion of their cargo, ostensibly to save themselves but in actuality for financial reasons. The trial, and Mansfield’s judgment, were taken up by abolitionists in their campaign against slavery, which would ultimately achieve statutory victory throughout the Empire in 1834.

Sagay links Mansfield’s decision to the presence of Belle in his household, and especially to her marital prospects after her receipt of a substantial bequest from her late father. Much of what Sagay posits about Belle’s private life at Kenwood is conjecture—her treatment within the family in the broadest sense, but particularly the material about her potential marriage, which takes on the cast of a Jane Austen novel. Important to that part of the plot is the Ashford family, headed by conniving Lady Ashford (Miranda Richardson). At first she thinks of her elder son James (Tom Felton), a loathsome bigot, as an appropriate match for Elizabeth until she finds that the girl will not inherit the Mansfield estate; then she supports her second, impecunious but pleasant son Oliver (James Norton) in his pursuit of Belle, willing to ignore the unfortunate color of her skin. All of this is clearly mere romance-novel folderol after the scene in which Belle meets (cutely, of course), John Divinier (Sam Reid), here presented as a rector’s son and aspiring lawyer who also happens to be an active abolitionist.

All this is invention; Belle eventually married a man named Divinier (no lawyer, it should be noted), but the notion that his arguments helped prod Mansfield to take his epochal decision in the Zong case has no basis in the record. Nor, in the broader sense, is the notion that Mansfield’s verdict was inspired by Belle’s personal circumstances persuasive. After all, in the earlier decision of Somerset v. Stewart (1772), he had already stated that slavery could be established in England only by positive law—a judgment that abolitionists took up in their campaign, and ultimately proved more germane to the success of their cause than that in the Zong case.

But if much of “Belle” is suspect from the historical perspective, the film is expertly put together from a dramatic one. It’s immeasurably helped by the performances, with the beautiful Mbatha-Raw making Dido a strong but vulnerable character, nicely matched by Gadon’s more flighty but touching Elizabeth. Reid is a stalwart, principled figure as Davinier, and Richardson and Felton are a perfectly odious pair, with Norton exuding befuddled pleasantness as the unfortunate third member of the Ashford trio. Much of the film’s success, however, depends on Wilkinson who endows Mansfield with exceptional nuance, and on his household confederates—Watson, who adds a delicious touch of wifely concern as Lady Mansfield, and Penelope Wilson as spinster Lady Mary, the Lord’s housekeeper, who turns out to be much less the imperious harpy she seems at first appearance.

The picture is also aided by the impeccable period dressing by production designer Simon Bowles, art directors Ben Smith and Claudia Campana, set decorator Tina Jones and costume designer Anushia Nieradzik, and the splendid location cinematography by Ben Smithard. Their work is of such high caliber that one barely notices how stodgily Asante often stages the action.

So if you appreciate the “Masterpiece Theatre” approach, “Belle” offers a fine big-screen example of it. And it touches on one of the most significant historical movements of the eighteenth century, even though toying with the facts in the process.