One of the annual irritations of the fall movie season is the arrival of the ubiquitous British costume drama. It's a subgenre so uniform in its style and quality that I'd swear the damnable things are machine-produced: start with a screenplay full of lovingly detailed archaic dialogue and a story that scoots through several years without bothering to let the audience know how much time is passing, spend most of the budget on opulent sets and gorgeous costumes, and then fill the cast up with British actors both well-established and upcoming, all of whom give appropriately restrained performances with clipped upper-class accents. Lastly, make sure that the director, cinematographer and editors don't do anything too interesting, else it might become accidentally thrilling to watch, and that is something that would be entirely unstuffy.

Given that framework, The Duchess fulfills all of the expectations it must, however much its ambitions are thus suffocated, and in this the film is very much like it protagonist. Actually, I'm not being fair; The Duchess is exceptionally decent for a costume drama, enjoying both the most luscious costumes of any film thus far in 2008, as well as a lead performance that surely ranks towards the top end of such things. These are not enough to keep the final product from being essentially banal (among other things, the direction is exceptionally flat), but they're compensations, for those who find themselves somehow unable to avoid seeing every last period piece that comes down the pike.

Keira Knightley plays the titular Georgiana Cavendish nΓ©e Spencer, Duchess of Devonshire, who in 1774, at the age of seventeen, made a politically outstanding but personally unhappy marriage to William, Duke of Devonshire (Ralph Fiennes), a cold and adulterous man. Quickly establishing herself as an influential figure in the progressive Whig party and England's leading fashionista, Georgiana first sought relief from her oppressive marriage in the form of an understanding best friend, Bess Foster (Hayley Atwell), who eventually moved in to the Duke and Duchess's estate and began sleeping with William; after that, Georgiana embarked on a love affair of her own with her long-time friend the young Whig Charles Grey (Dominic Cooper), later the second Earl Grey and still later Prime Minister of the United Kingdom. If the abstract version of that story (young woman enters a loveless marriage with an older member of the aristocracy, is beloved and beautiful, has affairs) calls to mind Georgiana's great-great-great-great-(great?)-grandniece Diana, rest assured that the filmmakers want very badly for everybody to notice that, and thus do I indulge them. Although, to be entirely honest, I'm not sure that this connection is in any way enlightening or important, outside of its use as a marketing tool.

So here's what's good about The Duchess: Keira Knightley. A charming young star who lurches between great roles and insipid ones, from Pride & Prejudice to Pirates of the Caribbean: At World's End, I would not typically say that she is a great actress according to one of the most important measures - she has never proven able to out-act a bad script - but when she is given the proper character to play, there are few better. And in The Duchess, she has been given a sublimely Knightleyan character to play, ending in what might - might - replace Cecila Tallis in Atonement as the greatest performance in her not-yet-decade-old career. Georgiana isn't terribly well-written, but she is well-written for Knightley, all repressed emotions and short-lived explosions of anger and lust. It's owing solely to Knightley's charisma that The Duchess has any emotional gravity whatsoever.

The rest of the cast doesn't do so bad themselves; though it's really the women's show all the way, with Atwell giving the movie a much-needed shot of life just when it's first starting to obviously sag, and Charlotte Rampling, as Georgiana's mother, Lady Spencer, plays the requisite "sharp old lady" role with real bite, sensitive and imperious in such a perfect balance that you don't notice when one shades into the other. As for the men, Fiennes lazily plays a role that he's played before -the smug British dick - and it's a bit of a disappointing slide back from his late run of fantastic performances in films like In Bruges, The Constant Gardner and even the last two Harry Potter entries, but he's not objectively bad; Cooper, whose career has consisted more in looking pretty than anything else, is the cast's only weak link, and happily he's been given the smallest role.

The other good thing: the clothes. I'd be tempted to argue that the sumptuous dresses and hats are the best and almost the only reason to see the film. Which we could almost claim is a generic requirement, sort of like saying that the gore is the only reason to see a horror film: the fanboys are happy either way. And by all means, Michael O'Connor's costume design is the sort that period film fanatics crave; even I, with my stated lack of affection for the form, couldn't help but goggle over the details and the richness, and the lovely way that they sit on Knightley's model-skinny frame.

Now for the bad thing: whatever awful things you think when the words "Masterpiece Theater" are spoken to you, these are the things which mark The Duchess for its entire running time. Pinched, airless, pompous, the movie clicks along through its plot at a pace paradoxically both rushed (from the first sign of pregnancy to birth in one cut - this happens twice) and halting. Director Saul Dibb, making his second feature, has only a rudimentary skill with the camera, and so scene after scene consists of the same four or five shots that all of these movies always use, over and over, slowly leeching all the energy out of the drama. Dibb has made a series of tableaux, and the nasty thing about tableaux is that they are lifeless. Even the several bedroom scenes (and to my memory, only the flawed but mesmerising Marie Antoinette spends so much time in aristocratic bedrooms, and at least in that movie they are energising), the parts of the film most prone to liveliness or at least salaciousness, are mannered and boring, even the film's left-field quasi-lesbian scene. It's just all so fucking respectable that it tips into drowsiness.