I met the inspector coming from the kitchen.

'Do you mind coming into the study with me, Doctor?' Inspector Davis unlocked the hall door and locked it again behind him. 'We don't want anyone to hear us. What's all this about blackmail? Is it Parker's imagination? Or is there something in it?'

'If Parker heard anything about blackmail, he must have been listening outside this door,' I replied.

'That's very likely. I didn't like his manner and when I questioned him again, he told me some story of blackmail.'

I told him the events of the evening.

'Most extraordinary story,' Davis said, when I had finished. 'And you say you couldn't see that letter on the study table when you found Ackroyd? Well, it gives us a motive for the murder.'

'Do you think that Parker himself might be the man we're after?'

'It looks likely. But keep it quiet until we've got all the evidence.'

He crossed over to Ackroyd's body in the armchair. 'The weapon ought to give us a clue.' He pulled the dagger out carefully from Ackroyd and put it in an empty flower vase on the mantelpiece. 'Quite a work of art.'

It was indeed a beautiful object. A narrow blade, and a beautifully decorated handle.

'Take a look at the handle. Fingerprints! I want to see if Mr Raymond can tell us anything about this dagger.'

We went back to the billiard room, where the inspector held up the dagger, still in the vase. 'Have you ever seen this before, Mr Raymond?'

'Why - that's the Tunisian dagger. It was given to Mr Ackroyd by Major Blunt.'

'Where was this kept?'

'In the silver table in the drawing-room.'

'What?' I exclaimed. 'When I arrived last night I heard the lid of the table being shut.'

I explained in detail.

'Was the dagger there when you were looking at the table?' the inspector asked.

'I don't remember noticing it.'

'We'd better ask the housekeeper,' remarked the inspector.

A few minutes later Miss Russell entered the room. 'Oh yes, the silver table was open,' she said, when the inspector had put his question. 'I shut the lid as I passed.'

'Can you tell me if this dagger was in its place then?'

'I can't say,' she replied.

'Thank you,' said the inspector.

Miss Russell left the room.