John Bayley's story of the last years of his wife Iris Murdoch's life, as she descended into the ever-tightening carapace of Alzheimer's dementia and died, interlarded with his memories of their most unlikely marriage of half a century's duration, has made two morbidly fascinating books and now a film - a film which won Academy Award nominations for Judi Dench and Kate Winslet, the aging and the young Murdoch; and an Oscar for Jim Broadbent as the aging Bayley.

The story is well known by now: the libertine Iris at Oxford, loving men and women as she wished, yet finding some unarticulated comfort in a relationship with the stammering, virgin John - she deflowers him when he least expects it - is beautifully handled in the film. Winslet is extraordinary as the glorious star of her world, openly sexual without being seductive, taking whomever she wished as lover, leading young John - a grievously unappreciated performance by Hugh Bonneville - into at least the imitation of an adult relationship.

The film cuts back and forth between the young and the old couple, much as Bayley's books do. We are present at the first intimations of Alzheimer's; and to watch Dench the actress let us in on them, finding the simplest actions, words, looks and hesitations as she starts down that inexorable path without a moment's self-pity, is to watch a great moment in film history. What is interesting to note, and disappointing I think, is that neither the books nor the film give us any sense of Murdoch as a writer. Her work, her prominence as author, is simply taken for granted; she is acclaimed, as we see at the beginning of the film, but her own artist's sensibility is unmentioned. We do not know her through her work.

The film sticks close to the books. Bayley was unsparing of himself, never pretending to a wit or strength or talent that he did not have; and Broadbent is faithful to that portrait, so much so that we find him faintly repugnant in his childish helplessness. But along with that goes a playful spirit, and the scenes of John and Iris swimming - the activity they loved to do together for fifty years - are utterly charming.

As Iris's mind closes down, with only an occasional flicker of perception left for John to cling to, we in the audience are awed by the seeming simplicity of Dench's work. The hardest job for an actor is to show nothing, and yet convey the meaning of that void. She does it with the skill of a genius. But she is not alone. Had I a vote in the Motion Picture Academy I would have given four Oscars to the principals here. Director Richard Eyre, who for the past twenty years has worked only in television, has treated this big-screen film as what it truly is: an intimate family drama. He's refused to make big directorial gestures or in any way to settle for melodrama. The notorious messiness of the household, which John is careful to describe in the books, is seen here only in the corners of shots; books, papers, magazines, journals litter the staircase of their little house. There isn't a chair without something on it. But it is all there for us as we look around the house, if we will notice.