The Duchess (12A)

It is, in its way, a horror story. An innocent 17-year-old girl is sent off by her mother to a stranger whose only interest in the match is the girl's ability to produce a male heir. Installed within the gilded cage of marriage, she discovers that the stranger, while nominally a member of her own tribe, is barely acquainted with the human race. Indeed, he has more affection for his dogs than for her.

Disgusted by her failure to give him a boy – she has two girls instead – the husband takes a mistress and ignores his wife, until the moment the latter finds true love. Then, asserting his lordly right, he breaks up his wife's affair and insists on her continuing as part of a humiliating ménage à trois. You could call the movie "Married Alive".

Or, you could make it a period picture and call it The Duchess. For these are the basic contours of the life of Georgiana Spencer, the Duchess of Devonshire, played for the screen by Keira Knightley in a succession of fabulous dresses and tottering wigs. Balanced against the horrors of her domestic life is the glittering spectacle of her public one: Georgiana was a much-loved celebrity of 18th-century England, and therefore a certain winner for a 21st-century biopic.

Director Saul Dibb, working with a script adapted from Amanda Foreman's best-selling biography, understands the meat-and-drink of the genre – grand perspectives, social colour, formal propriety – yet never loses sight of the human drama. Dibb came to prominence with Bullet Boy (2004), in which Ashley Walters' young offender emerges from prison into a cycle of violence amid the badlands of high-rise Hackney. Different setting, different century – but the same problem of youth trapped by class and expectation.

Georgiana's Gainsborough beauty and extravagant clothes make her the cynosure of fashionable society, but it is her taste for gambling, drinking and politics that sets her apart. There's a shot of her framed at the head of a dinner table and trading witty repartee with the Whig party leader Charles James Fox (Simon McBurney). The camera eventually turns and we look down the table from her perspective: every other diner is a man. It's a nice way of signalling her ease within the crucible of power.

If this suggests that Georgiana was a woman out of time, advice from her mother (Charlotte Rampling) about conjugal duties locates the girl very much as of her age. "It can be a bother," the mother concedes, but she'll get through it with "patience, fortitude and resignation". Not bad advice, but it still hardly equips her to deal with the husband in question. Ralph Fiennes, thin-lipped and fat-bellied, plays the Duke as the coldest of cold fish, as distant and bored as his wife is gregarious and convivial.

There is a kind of enigma about this, since His Grace chose the lady for his wife. The Duke, someone remarks, "is the only man in England not in love with the Duchess". While she floats through the glistening social aquarium of high Georgian society, she seems safe, but the want of a husband's love and interest gradually become impossible to ignore. This becomes positively unendurable once the Duke takes Lady Bess Foster (Hayley Atwell) as his mistress, a doubly cruel choice inasmuch as the lady was Georgiana's closest friend.

Dibb illustrates this in his composition of the ménage at dinner, a static shot of the Duke and Duchess at either end of the long dining table, with Bess plumb in the middle. It's like the William Quiller Orchardson painting of the unhappy Victorian marriage, the young wife at one end, the doddery husband at the other. But here it's not age that separates them – it's an assertion of aristocratic might. "Loyalty and a male heir" is what he demanded of her; what she could demand of him was apparently left unspecified.

The humiliations of the marriage culminate in tragedy, which Knightley carries off pretty convincingly. She has eliminated much of the pouting and petulance that characterised her early screen roles, and she wears the costumes with grace (the wigs look too heavy for her slender neck). Her doomed affair with rising but impecunious politician Charles Grey (Dominic Cooper) carries rather less than the pathos intended; Cooper has a certain froggy handsomeness but not yet sufficient personality to give us the required jolt.

Fiennes, on the other hand, is chillingly good as the Duke, a detailed embodiment of snake-blooded calculation and a charmless will to power. Towards the end he hints that he might not be quite the brute he seems, and, in a deliberate allusion to another high-born adulterer, talks of his own vague understanding of what love means.

The film, to its credit, does not exploit the links between Georgiana and a later Spencer girl, though the idea of Diana as spiritual heir is implicit. The distributors, however, have no such delicacy; the film's trailer simply cuts between the two lovelorn Spencers. People's princess, innit. And the tag ("There were three people in her marriage") creates a tawdry echo of a certain TV interview.

It's all in the name of "relevance", that nervous modern reflex which holds that cinema audiences won't respond to the plight of an 18th-century woman unless they can see elements of their own experience reflected in it. Can we not just enjoy what's on the screen without being spoon-fed a specious modern analogy? Dibb and his team – especial plaudits to Michael Carlin's production design and Michael O'Connor's costumes – have worked honourably to persuade us that we can.