Sturry gave him a quelling look, and replied with meticulous politeness: ‘Mr Joseph, Inspector, desired me to enquire whether you, and the Other Policeman, will be requiring luncheon. If this should be the case, a Cold Collation will be served in the morning-room.’

‘No, thanks,’ said Hemingway, who had no opinion of cold collations at midwinter.

Sturry bowed slightly. His arctic gaze took in the position of the chair which the Sergeant had used to enable him to reach the knives on the wall, and travelled upwards. He acknowledged the disappearance of one of the pair of knives by a pronounced elevation of the eyebrows, and moved forward to restore the chair to its place against the wall. He then plumped up a couple of cushions, looked with contempt at the partially dismantled Christmas tree, and at last withdrew.

The Sergeant, who had been watching him with considerable disfavour, said: ‘I don’t like that chap.’

‘That’s only inferiority complex,’ said Hemingway. ‘You didn’t like being called the Other Policeman.’

‘Snooping round,’ said the Sergeant darkly. ‘He saw the knife had gone all right. He’ll spread that bit of news round the house.’

‘Then we may get some interesting reactions,’ responded Hemingway. ‘Come on! We’ll take the knife back to headquarters, and get a bit of dinner at the same time. I want to think.’

He was unusually silent during the hot and substantial meal provided by the cook at the Blue Dog inn; and the Sergeant, respecting his preoccupation, made no attempt to converse with him. Only when the cheese was set before them did he venture to say: ‘I’ve been thinking about that weapon.’

‘I haven’t,’ said Hemingway. ‘I’ve been thinking about that locked door.’

‘I don’t seem to get any ideas about that,’ confessed the Sergeant. ‘The more I think about it the more senseless it seems.’

‘There must have been a reason for it,’ said Hemingway. ‘A pretty strong one, too. Whoever murdered Nathaniel Herriard, and locked the door behind him, was taking the hell of a chance of being caught in the act. He didn’t do it for fun.’

‘No,’ agreed the Sergeant, thinking it over. ‘It looks as though you’re right there. But what reason could he possibly have had?’

Hemingway did not answer. After a few moments, the Sergeant said slowly: ‘Supposing the murdered man didn’t lock the door himself, in the first place? We’ve no proof that he did, after all. I was just wondering… If the murderer walked into the room, and locked the door behind him –’

‘Old Herriard would have kicked up a rumpus.’

‘Not if it had been his nephew he wouldn’t. He might have thought Stephen wanted to have a straight talk with him, without the valet’s coming in to interrupt them.’

‘Well?’ said Hemingway, showing a faint interest.

‘Well, Stephen, or someone else, killed him. You remember the valet telling us that he came along, and tried the door, and found it locked? Suppose the murderer was still in the room then?’