Barely more than a decade after that tragic dash through Paris and the unhinged, hysterical carnival of lamentation that followed, it is no great surprise that Princess Diana continues to cast a shadow over popular culture, particularly in the country where she once seemed destined to be queen. It is, nonetheless, disappointing to detect that elements of her tawdry, strip-mined melodrama have been slipped into Saul Dibb’s new film “The Duchess,” which arrives in theaters Friday. The British director gives what should have been a perfectly respectable biopic of Georgiana, an 18th-century Duchess of Devonshire, contemporary resonance it neither needs nor deserves.

Yes, Diana was the duchess’s great-great-great-great-niece and, yes, both women weathered marriages that were indeed (to borrow a word) “crowded,” but neither genealogy nor (very) superficially similar matrimonial difficulties are good reasons to blend their (very) different stories. The lure of the box office is, I suppose, to blame. Diana still sells.

Very loosely based on Amanda Foreman’s clever, immaculately researched, and enthralling biography of Georgiana, Mr. Dibb’s movie has taken the story of one of the most fascinating Englishwomen of her epoch — a celebrated socialite and political campaigner — and transformed it into a big-budget blend of Lifetime television, Masterpiece Theatre, and Diana Spencer tribute movie. Thus, the young duchess (Keira Knightley) speaks in the soft Sloane tones more typically associated with Lady Di in her early years than with the Georgian grandee she is meant to be playing. Meanwhile, Ralph Fiennes, in a subtle, show-stealing portrayal of the duchess’s cold, buttoned-up, and older husband, manages to punctuate his performance with very specific hints of Prince Charles’s lugubrious tics, mannerisms, and phraseology — hints that will make a British audience, at least, shudder or snigger, depending on mood.

For the most part, however, this film’s sins are of omission. Georgiana may have been a famed fashion icon, but she was also a genuinely effective power broker, a fiercely intelligent woman known as much for her Whiggery as for her truly remarkable wigs, an angle the filmmakers have downplayed in favor of crowd-pleasing emotional drama and roller-coaster marital crises. It’s typical that the movie ends on a note of gently accommodating family reconciliation, concluding its narrative at a point that may make some sort of soap-operatic sense, but is well before Georgiana’s final period of political prominence. To be fair, at various times we do see the duchess electioneering, and at others she’s shown hanging out with Charles James Fox (a potato-faced Simon McBurney, sufficiently wily, sufficiently charming, insufficiently louche) and the rest of his clique, but, taken as a whole, the film leaves the clear impression that the duchess’s political role was primarily ornamental. In reality, it was substantially more than that, no small achievement more than a century before female suffrage.

Rather more flatteringly for the duchess, we are not told, except through the most oblique of references, the extent to which her love of gambling (one of the main aristocratic pastimes of that period) became an addiction, bringing in its wake losses that might have brought a blush to the Lehman Brothers’s mortgage bond team and which, in part, explained why the poor duke might sometimes have looked a little pained. The reason for this particular omission is probably the filmmakers’ wish to present cinemagoers with a suitably sympathetic romantic heroine (so far as they reasonably could, given the tricky historical record). To show her losing tens of thousands at the faro table wouldn’t really have done the trick.

Similarly, the duchess’s love life (something she pursued with a splendidly 18th-century gusto) is mainly reduced to misery at the hands of her unfeeling husband (that was true enough, alas), a series of harmless flirtations, a not-quite seduction by the woman who goes on to become the duke’s live-in mistress, and then one great romance with a future prime minister, Charles Grey (Dominic Cooper, doing his unconvincing best to channel the BBC’s Mr. Darcy). The truth was considerably busier, rather more complicated, and much more interesting.

If Georgiana’s biography has been prettied up, so has the country in which she lived. Eighteenth-century England was a grubby, smelly, uncomfortable place. Even its grandest houses were just a pace or two from squalor and were, for the most part, none too clean themselves. The same could be said of their inhabitants, not to mention those unfortunate enough to live beyond ducal walls. The beautifully filmed England of “The Duchess” (courtesy of cinematographer Gyula Pados) is, by contrast, immaculate, a land of lush landscapes, Augustan charm, and gorgeous Palladian magnificence. It bears about as much resemblance to the real thing as Marie Antoinette (a friend of Georgiana’s, not that you’d know it from this movie) did to the simple shepherdesses she occasionally pretended to be.

No matter. As a backdrop to what is, in essence, a well-crafted, well-acted, period romance, this prettily stage-set, sceptr’d isle will do just fine. We’ll leave the slums, the stench, and, for that matter, the disease that was later to wreck the lovely Georgiana’s looks to some other, more realistic film.

But if you allow yourself to overlook the historical inaccuracy, the faint feminist subtext, and the forced, tiresome parallels with the Windsors’ domestic disasters, “The Duchess” can be fun. So why not take a break from Wall Street worries and wallow instead in an hour or two of spectacle, splendor, and sentimentality?

Aided by landscape, architecture, and costume, “The Duchess” looks terrific and the script does its best, too, helped along by a cast stronger than this film probably deserves. Mr. Fiennes may steal the show, but as Lady Spencer (Georgiana’s mother), a matriarch who combines strong maternal affection with a steely sense of dynastic obligation, the perennially formidable Charlotte Rampling dominates every scene in which she appears. By comparison, Ms. Knightley was bound to struggle, but with her strangely old-fashioned beauty, she at least looks the part, and the pathos she successfully brings to her performance reinforces the aura of victimhood without which no romantic heroine is complete. In such a shamefully enjoyable film, what more could one ask?