Iris Murdoch, Professor of Philosophy and celebrated author and her husband, Professor of Literature John Bayley, are guests-of-honour at a banquet at Somerville College. Iris Murdoch gives a brilliantly witty speech and finishes with an Irish folk song, "The Lark in the Clear Air". As she begins to sing, John Bayley's thoughts wander back to the evening when Iris was first introduced to him at a dinner and he was so taken with her that he choked on his wine. Even back then, she always had a ready wit. Over the next few weeks, John is often reminded of the past, because of Iris' health, which is beginning to worry him. A medical examination reveals that Iris Murdoch is suffering from Alzheimer's disease. Iris' condition worsens and before long, she doesn't even recognise John any more. Although her illness is sometimes too much of a strain, even for John, the memory of their powerful love for each other nevertheless remains unforgettable.