‘Extraordinarily kind of you,’ said Alleyn grimly. ‘Is there any exit from the buffet other than the one into the hall?’

‘Yes, there is. A door that gives on to the back stairs down to the basement.’

‘So it’s conceivable that Dimitri might have gone out into the street that way?’

‘Yes,’ agreed Fox. ‘It’s possible, all right. And come back.’

‘He would have been away at least forty minutes,’ said Alleyn, ‘if he’s our man. If, if, if! Would he be able to get hold of a topper? The murderer wore one. What would he say to Bunchy to persuade him to give him a lift? “I want to talk to you about blackmail?” Well—that might work.’

‘For all we know,’ said Fox, ‘it may not have been any of the guests or Dimitri.’

‘True enough. For all we know. All the same, Fox, it looks as if it was. It’s not easy to fit an outsider into what facts we’ve got. Try. An unknown in full evening dress wearing an overcoat and a top-hat stands outside Marsdon House waiting for Lord Robert to come out and on the off-chance of getting a lift. He doesn’t know when Lord Robert will leave, so he has to hang about for three hours. He doesn’t know if he’ll get a chance to speak to Lord Robert, whether Lord Robert will leave in a party or alone, in a private car or a taxi. He doesn’t know a heavy mist is going to crawl over London at one o’clock.’

‘He might have just happened to come up,’ said Fox and added immediately: ‘All right, all right, sir. I won’t press it. We’ve got plenty to go on from inside and it’s a bit far-fetched, I will allow.’

‘The whole thing’s too damn far-fetched, in my opinion,’ said Alleyn. ‘We’re up against a murder that was very nearly unpremeditated.’

‘How do you make that out?’

‘Why, Fox, for the reasons we’ve just ticked off. Lord Robert’s movements could not be anticipated. I have just learned that he had intended to leave much earlier with his sister, Lady Mildred Potter, and Miss Troy.’

‘Miss Agatha Troy?’

‘Yes, Fox.’ Alleyn turned aside and looked out of the window. ‘She’s a friend of the family. I’ve spoken to her. She’s here.’

‘Fancy that, now,’ said Fox comfortably.

‘I think,’ continued Alleyn after a pause, ‘that when the murderer went out from the lighted house into that unwholesome air he perhaps knew that Bunchy—Lord Robert—was returning alone. He may have seen him alone in the hall. That’s why your little list is important. If the man was Dimitri he went out with the deliberate intention of accomplishing his crime. If it was one of the guests he may have made up his mind only when he caught a glimpse of Bunchy standing alone in the mist, waiting for a taxi. He may have meant to threaten, or reason, or plead. He may have found Bunchy obdurate, and on an impulse killed him.’