Iris, Richard Eyre (15)

Oddly, for a biopic of a Booker Prize-winning novelist and giant intellect, you get to see a lot of Kate Winslet's breasts in this movie, wobbling softly and palely like two blancmanges being borne up to High Table on a trolley.

Over the opening credits, a naked, nymph-like Winslet swims up through a suspiciously milky Isis and almost bumps her boobies against the lens. But then, Iris's "greatest talent was for life", wasn't it? So Winslet gives us a yomping, skinny-dipping hoyden with a Joan of Arc hairstyle and big brows, and Judi Dench is an old dear with a pebble fetish.

This film is not interested in Murdoch's ideas, her imagination. Of her enchanters and her maidens, the doubting priests, malign lovers and flawed philosophers of her fictions, there is not a trace. The film, based on John Bayley's memoirs Iris and Iris and the Friends, unfortunately compounds the notion that it wasn't her books that mattered, but her final transfiguration into sheer goodness and childishness – her transcendence of the word. Film, so often indifferent to writing, here seems positively hostile to it.

There isn't much sense – beyond the heroically filthy Oxford house – that we're looking at a pair of venerable academics, either. The establishing dialogue is mostly lame, though there's a marvellous scene in a supermarket when the expert on Plato, having been sidetracked by the desire and pursuit of whole-grained mustard, becomes momentarily transfixed by the notion of a "bag for life".

The film see-saws back and forth, from old to young, senile to brilliant, Dench to Winslet. Hugh Bonneville as the young Bayley and Jim Broadbent as the old one are more successful in giving the impression that they are playing the same person; but then Dench and Winslet actually aren't. Nearly everything that made Iris Iris was being burgled, and Dench's depiction of this is undeniably moving.

Winslet's Iris is ravishing in a red dress at a college dance, daring and sexually promiscuous (more breasts). Her long, cool, speculative stare when her stammering swain wonders jokily whether she sleeps with all those Eton-cropped female admirers is priceless. I was interested to learn that in post-War Oxford it was easy to wander into people's college rooms and spy on them having sex. (That tired convention where the actors have to leave the door open behind them for the camera crew to get in has never been so irritating.)

There comes a point in the seduction when you lattice your fingers over your eyes and hope you won't be forced to watch Murdoch and Bayley, even in such attractive avatars, having sex. We're spared that, at least. The real stripping comes later, when the doctors fall upon this interesting case-study and condescendingly call her by her first name. "That's Dame Iris Murdoch to you," you want to shout.

But there's no denying this is Dench's triumph, as she rubs and rubs away at the characterisation until there is hardly anything left, just a loving spirit. There ought not to be a dry eye in the house when an ogre-sized taxi-driver coaxes her down from her perch on the stairs for her final journey to a home for Alzheimer's sufferers. Who should pop up at the hospice but Father Ted's satanic Mrs Doyle? It's a hideous miscalculation. Pauline McGlynn gurns and cackles so malevolently that it's no surprise Murdoch's dead in under a minute.

A few days after seeing the film, I met an academic who had known both Bayley and Murdoch. I mentioned that Murdoch would soon be famous for being a victim – for being the wife of the long-suffering John Bayley – rather than for her novels. "Oh, the work will survive," he chuckled. Then he went on to say how difficult she was and how marvellous Bayley had been: "He deserves it." I couldn't help thinking: wife looks after sick husband – so what? Husband looks after sick wife – give the man a medal!

Suzi Feay is Literary Editor of The Independent on Sunday