Then her mind veered sharply to the consideration of his sister, and she began to feel that she was living in a world of nightmare. Would Paula be capable of stabbing to death an old man who loved her, merely for the sake of a part in an unknown dramatist’s play? She didn’t know. She had no clue to Paula either; she only knew her as an urgent, unbalanced young woman, always obsessed by the idea of the moment.

Yes, but although Paula had been seen at Nat’s door, how had she contrived to get into a locked room, or, more difficult still, to lock it behind her? Mathilda had no knowledge of the means by which doors could be locked and unlocked from the wrong side, but she knew that there were such means. Yet it seemed unlikely that Paula could have employed any of them, for how could she have acquired the necessary tools?

This led to the question, were they in it together, this odd, frustrated brother and sister? It was too diabolical: Mathilda shied away from the thought, miscued, and straightened herself, saying with a breathless laugh: ‘Oh, damn! You’ll run out now!’

‘I wonder what that Scotland Yard man’s up to?’ Stephen said restlessly.

‘Trying to trace the person who handled that dagger,’ she suggested.

‘He won’t do that.’

The confidence in his tone startled her. She looked at him almost fearfully. ‘How do you know?’

He bent over the table for his shot. ‘Bound to have wiped the finger-prints off it,’ he replied. ‘Any fool would know enough to do that.’

‘I suppose so,’ she agreed. ‘Whoever did it was pretty ingenious. How could anyone have got into the room? And how was the door locked afterwards?’

‘Hell, how should I know?’

‘How should any of us know?’ she asked. ‘This isn’t a house full of crooks! We’re all ordinary people!’

‘Even though one of us is an assassin,’ interjected Stephen.

‘True; but although I’m not personally acquainted with any assassins –’

‘You are personally acquainted with one assassin, my girl.’ He saw how quickly her eyes leaped to his, and added, with one of his mocking smiles: ‘Since someone in this house is one.’

‘Of course,’ she said. ‘It’s rather hard to realise that. I was going to say that I’ve always imagined that a murderer could be quite an ordinary person. Not like a confirmed thief, I mean. Which of us, for instance, would know how to open a locked door? Of course, I suppose one of the servants might be a crook, but I don’t quite see why any of them should have wanted to murder Nat. They none of them gain anything by his death.’

‘True,’ said Stephen uncommunicatively.

‘Could there be anything in that idea of Valerie’s? Is there a sliding panel, or anything of that kind?’

‘I’ve never heard of it.’

She sighed: ‘No; it does seem rather fantastic. But someone got into that room somehow, and if it wasn’t through the door or the window, how was it?’