'The pathologist can usually estimate the time of death with reasonable accuracy. And I knew she'd be found that night. I knew that Alex had promised to visit her when he got back from the power station. He was on his way to the cottage when he met Adam Dalgliesh. And now, I think, you know it all, except for the Bumble trainers. I came through the back gardens at the Old Rectory late on Sunday afternoon. I knew that the back door would be open and it was the time when you would be having high tea. I had a bag with me with a few items of jumble in case I was seen. But I wasn't seen. I took soft shoes, easy to wear, a pair that looked roughly my size. And I took one of the belts.'

But there was one more question to ask, the most important of all. Meg said: 'But why? Alice, I have to know. Why?'

'That's a dangerous question, Meg. Are you sure you really want the answer?'

'I need the answer, need to try to understand.'

'Isn't it enough that she was determined to marry Alex and I was determined that she shouldn't?'

'That isn't why you killed her. It can't be. There was something more than that, there had to be.'

'Yes, there was. I suppose you have a right to know. She was blackmailing Alex. She could have stopped him getting that job, or, if he had got it, could have made it impossible for him to function successfully. She had the power to destroy his whole career. Toby Gledhill had told her that Alex had deliberately held up publishing the result of their research because it might have prejudiced the success of the inquiry into Larksoken's second reactor. They discovered that some of the assumptions made in generating the mathematical models were more critical than had been thought. People opposing the building of the new PWR at Larksoken could have exploited it to cause delay, whip up fresh hysteria.'

'You mean that he deliberately falsified the results?'

'That's something he's incapable of doing. All he did was to delay publishing the experiment. He'll publish it within the next month or two. But that's the kind of information which, once it got into the press, would have done irreparable harm. Toby was almost prepared to hand it over to Neil Pascoe but Hilary dissuaded him. It was far too valuable for that. She meant to use it to persuade Alex to marry her. She faced him with the knowledge when he walked home with her after the dinner party and late that night, he told me. I knew then what I had to do. The only way he might have been able to buy her off was by promoting her from Acting Administrator to Administrative Officer of Larksoken, and that was almost as impossible for him as deliberately falsifying a scientific result.'

'You mean he might actually have married her?'

'He might have been forced to. But how safe would he have been even then? She could have held that knowledge over his head until the end of his life. And what would that life have been, tied to a woman who had blackmailed him into marriage, a woman he didn't want, whom he could neither respect nor love?'

And then she said in a voice so low that Meg only just heard it, 'I owed Alex a death.'

Meg said: 'But how could you be sure, sure enough to kill her? Couldn't you have talked to her, persuaded her, reasoned with her?'

'I did talk to her. I went to see her on that Sunday afternoon. It was I who was with her when Mrs Jago arrived with the church magazine. You could say that I went to give her a chance of life. I couldn't murder her without making sure that it was necessary. That meant doing what I'd never done before, talking to her about Alex, trying to persuade her that the marriage wouldn't be in either of their interests, to let him go. I could have saved myself the humiliation. There was no argument, she was beyond that. She was no longer even rational. Part of the time she railed at me like a woman possessed.'

Meg said: 'And your brother, did he know about the visit?'

'He knows nothing. I didn't tell him at the time and I haven't told him since. But he told me what he planned; to promise her marriage and then, when the job was secure, to renege. It would have been disastrous. He never understood the woman he was dealing with, the passion, the desperation. She was a rich man's only child, alternately overindulged or neglected, trying all her life to compete with her father, taught that what you want is yours by right if you've only got the courage to fight for it and take it. And she had courage. She was obsessed by him, by her need for him, above all by her need for a child. She said that he owed her a child. Did he think she was like one of his reactors, tameable, that he could let down into that turbulence the equivalent of his rods of boron-steel and control the force which he'd let loose? When I left her that afternoon I knew I had no choice. Sunday was the deadline. He had arranged to call at Thyme Cottage on his way home from the power station. It was fortunate for him that I got to her first.

'Perhaps the worst part of all was waiting for him to come home that night. I daren't ring the power station. I couldn't be sure that he would be alone in his office or in the computer room, and I had never before telephoned him to ask when he would be home. I sat there and waited for nearly three hours. I expected that it would be Alex who would find the body. When he discovered that she wasn't in the cottage the natural move would be to check at the beach. He would find the body, telephone the police from the car and ring home to tell me. When he didn't I began to fantasize that she wasn't really dead, that somehow I'd bungled it. I pictured him desperately working on her, giving her the kiss of life, saw her eyes slowly open. I turned off the lights and moved to the sitting room to watch the road. But it wasn't an ambulance that arrived, it was the police cars, the paraphernalia of murder. And still Alex didn't come.'

Meg asked: 'And when he did?'

'We hardly spoke. I'd gone to bed; I knew I must do what I would normally have done, not waited up for him.

He came to my room to tell me that Hilary was dead and how she had died. I asked "The Whistler?" and he answered, "The police think not. The Whistler was dead before she was killed." Then he left me. I don't think either of us could have borne to be together, the air heavy with our unspoken thoughts. But I did what I had to do, and it was worth it. The job is his. And they won't take it away from him, not after it's been confirmed. They can't sack him because his sister is a murderess.'

'But if they found out why you did it?'

'They won't. Only two people know that and I wouldn't have told you if I couldn't trust you. On a less elevated level, I doubt if they'd believe you in the absence of confirmation from another witness; and Toby Gledhill and Hilary Robarts, the only two who could give it, are dead.' After a minute's silence, she said: 'You would have done the same for Martin.'

'Oh no, no.'

'Not as I did. I can't see you managing to use physical force. But when he drowned, if you could have stood on that river bank and had the power to choose which one should die and which live, would you have hesitated?'

'No, of course not. But that would have been different. I wouldn't have planned a drowning, wouldn't have wanted it.'

'Or if you were told that millions of people would live more safely if Alex got a job which he is uniquely capable of filling but at the cost of one woman's life, would you hesitate then? That was the choice which faced me. Don't evade it, Meg. I didn't.'

'But murder, how could it solve anything? It never has.'

Alice said with sudden passion: 'Oh, but it can, and it does. You read history, don't you? Surely you know that.'

Meg felt exhausted with weariness and pain. She wanted the talking to stop. But it couldn't. There was still too much to be said. She asked: 'What are you going to do?'