ONE SUNNY Wednesday afternoon last month, Mary McEwan and her husband, Donald, a retired teacher, packed their bags for the drive home after a two-day break at a country hotel. They decided to save themselves a shopping trip later in the week by stopping off at Sainsbury's in King's Lynn before heading back to Binham, the pretty village near the north Norfolk coast where they live.

Mary asked her husband if he wanted to come round the shop with her. But he decided, as usual, to sit down in the store and wait for her. That was the last time she saw him. Donald, who suffers from Alzheimer's disease, has disappeared.

'I could see him each time I came down the aisles,' Mary said. 'My last memory of him was he looked very happy. He was laughing at something, he was obviously watching something, probably children. Young children took his eyes these days, he got a lot of pleasure out of watching them. . . .'

When she realised Donald had vanished, Mary thought he might have just wandered off. 'You see, the logic had gone; he wouldn't think 'She's been a long time, I'll go and look for her'. And of course I just dropped everything.'

She alerted the store detective and stopped a police car outside. The police issued a description of Donald - known as 'Mac' - a 74-year-old man of slight build with white hair, wearing a light green sweater over a mustard yellow polo shirt, fawn trousers and brown brogues. He was wearing a silver identity bracelet but carried no money because he was worried about losing it. The police drove Mary around the town, searching for him. 'There was no time loss . . . but he could have gone in umpteen directions.'

The family and police organised ground and aerial searches. But 21 days later, Donald is still missing.

'It's a terrible thing,' said his wife, a kind and intelligent woman who is struggling to maintain hope. 'It's worse not knowing, not being able to say goodbye or close the book. And it's a good book, that's why I hope against hope they will find him.'

The McEwans met at grammar school in Barking, Essex, when Donald was 15 and Mary was 12. A black-and- white portrait photo taken when Donald was 23 shows a handsome, sensitive face. In more recent family snaps, he is smiling; he looks fit and cheerful. They have two sons, David and Andrew, and will have been married for 50 years on 16 August. 'We'd planned a great celebration. He knew about that, he went along with it . . . probably when he was younger and more himself he would have said 'Oh, I don't know' and only accepted it reluctantly.'

Donald, a handsome man with 'a very Scottish face', is not obviously ill at first glance. 'When he first left that shop, he would have walked quite purposefully,' Mary said. 'He did not shuffle along like an old man, the Alzheimer's had not reached that sort of stage with him. He would know his name, but I don't know that he would know his address. He would miss me physically because he always wanted me around.'

Donald can hold a conversation, but might lose his train of thought. He can still tie a tie or cravat, but Mary has to dress him because he might put his shoes on before his trousers.

'Mac could still read, but he couldn't carry it to the next page. He watched television, though, old films, simple things, and he liked music,' Mary said. 'I never had the torture of him saying to me: 'Who are you?' A lot of people get like that.'

One day, shortly before he retired, Donald told his wife he had forgotten the answer to a pupil's question; to cover up, he told the class to look up the answer themselves - 'he always did put on a good front'. With hindsight, that was the first sign of Alzheimer's, a progressive brain disorder that causes dementia. Mary has noticed a serious deterioration in his condition only during the past 18 months.

'The dreadful thing is you're glad they can't see themselves. My husband would have hated it, he's a typical Scot, quite a proud man. . . . There was a lot about him that had gone, but there were flashes of his old self. It happened very gradually . . . and you have to look back and think about what has changed. I once heard Jonathan Miller saying Alzheimer's is a beholder's disease, and it truly is . . . it's so cruel.'

Half a million Britons suffer from the disease; for those who care for them, the responsibility is enormous, said Clive Evers, information officer for the Alzheimer's Disease Society. 'We tell carers to carry on with their normal day-to-day lives as best they can for as long as they can, because it is a very demanding task.'

The society, said Mr Evers, receives two or three phone calls a month from carers whose charges have gone missing, but it may happen much more often. 'It's very difficult to maintain absolute vigilance or 24-hour contact.'

Donald has been missing twice before, once in London for several hours, a second time in his village. 'You might well say how did I come to lose him? I think that, too, but I try not to feel guilty,' said Mary, 'because I've done it (taken him shopping) hundreds of times. He sits down and I keep an eye on him. He used to be quite happy chatting because there are always old people on those seats.'

The second time her husband went missing, 'I didn't feel so bad because Binham is safer. We had the whole village out looking. The police picked him up just before 8am in a field. I think he'd slept in a hedge. He was all bedraggled, but he was all right. I said to him: 'I've been frantic. Weren't you frightened out there on your own?' and he said no, he wasn't'

Mary sighed. 'Generally speaking, he wasn't frustrated, which is what I was always frightened of. . . . There were so many things he couldn't do any more. He loved reading, and that had gone . . . and he was very good with his hands.' On the walls of their home, a converted farm building, hang pictures in the heavy gilt frames Donald enjoyed restoring.

Mary catches herself speaking of Donald in the past tense, but it is because of the Alzheimer's: he is no longer the man he once was. 'Mac was a good storyteller and a good teacher - a disciplinarian when he came out of the Army. He taught everything; he was particularly good at art, but his subjects were Latin, French, English and History. He didn't go to university after the war because we were married and had a baby, so he couldn't afford it. Years later, he regretted it.'

Donald spent six years in the Army during the Second World War; his military training gives his wife a glimmer of hope that he might be able to survive if he is sleeping rough. 'The 40 years of teaching had gone, he never referred to it, but the wartime, the soldiers, he would talk about Burma as though it were yesterday,' she said. 'And we hope that the basic instincts, the survival instincts, are still there.'

But Mary knows the chances of finding her husband alive are not good. As Mr Evers said: 'Somebody with moderate dementia will have basic needs for food and water and will almost certainly seek them out but won't pursue them in a rational way.'

The King's Lynn police are very concerned. Acting Inspector David Chilvers said: 'Realistically, we don't think we'll find him alive. But until we know otherwise, we'll keep looking.' They have circulated details to neighbouring forces and registered him on the nation-wide list of missing persons.

'We've had quite a bit of local publicity and 26 possible sightings, from March in Cambridgeshire to the Norfolk Broads,' said Insp Chilvers. 'We look at it every day and try to make an effort to resolve it. But this is a rural area and it presents us with considerable problems with regard to physical searches. We need a starting point and we don't have one.'

'I don't cry very easily. I have cried at night,' Mary said. 'I said to Andrew: 'I feel so terrible that he will die on his own, with no one to hold his hand' - and he said: 'We all die alone'. I suppose we do. But he needed me more than he used to.

'He was a free spirit and he liked going out. And he was very happy that day, and I'm sure he didn't do it deliberately. My sons were saying he'd have a good laugh if he knew what was going on, all this notoriety, and he would, too. And Andrew said to me: 'Mother, there's something so archetypal about it'. Mac was a romantic and this is epic.

'I try to keep busy because then it's not so bad. But it is a weight and an anguish. . . . For a long, long time you're protective towards them, it's almost like a child.'

Her distress is compounded by the practicalities of Donald's disappearance. Mary told the Social Security office, which stopped his pension. 'I couldn't go on taking it, knowing he wasn't here . . . but if I didn't have savings of my own, it would be a bit grim. I've not got much hope, not unless someone has taken him in. The thing that hurts most is the torture of what really did happen to him and also whether they will find him. I hope to God they do.'

Anyone who thinks they may have seen Donald McEwan should contact King's Lynn police on 0553 691211.

The Alzheimer's Disease Society can be contacted on 081-675-6557.

(Photographs omitted)