‘With the greatest respect, Lord Mayfield, you imagined you saw him. The shadow cast by the branch of a tree deceived you. The fact that a robbery occurred naturally seemed a proof that what you had imagined was true.’

‘Really, M. Poirot, the evidence of my own eyes—’

‘Back my eyes against yours any day, old boy,’ put in Sir George.

‘You must permit me, Lord Mayfield, to be very definite on that point. No one crossed the terrace to the grass.’

Looking very pale and speaking stiffly, Mr Carlile said:

‘In that case, if M. Poirot is correct, suspicion automatically attaches itself to me. I am the only person who could possibly have committed the robbery.’

Lord Mayfield sprang up.

‘Nonsense. Whatever M. Poirot thinks about it, I don’t agree with him. I am convinced of your innocence, my dear Carlile. In fact, I’m willing to guarantee it.’

Poirot murmured mildly:

‘But I have not said that I suspect M. Carlile.’

Carlile answered:

‘No, but you’ve made it perfectly clear that no one else had a chance to commit the robbery.’

‘Du tout! Du tout!’

‘But I have told you nobody passed me in the hall to get to the study door.’

‘I agree. But someone might have come in through the study window.’

‘But that is just what you said did not happen?’

‘I said that no one from outside could have come and left without leaving marks on the grass. But it could have been managed from inside the house. Someone could have gone out from his room by one of these windows, slipped along the terrace, in at the study window, and back again in here.’

Mr Carlile objected:

‘But Lord Mayfield and Sir George Carrington were on the terrace.’

‘They were on the terrace, yes, but they were en promenade. Sir George Carrington’s eyes may be of the most reliable’—Poirot made a little bow—‘but he does not keep them in the back of his head! The study window is at the extreme left of the terrace, the windows of this room come next, but the terrace continues to the right past one, two, three, perhaps four rooms?’

‘Dining-room, billiard-room, morning room and library,’ said Lord Mayfield.

‘And you walked up and down the terrace, how many times?’

‘At least five or six.’

‘You see, it is easy enough, the thief has only to watch for the right moment!’

Carlile said slowly:

‘You mean that when I was in the hall, talking to the French girl, the thief was waiting in the drawing-room?’

‘That is my suggestion. It is, of course, only a suggestion.’

‘It doesn’t sound very probable to me,’ said Lord Mayfield. ‘Too risky.’

The Air Marshal demurred.

‘I don’t agree with you, Charles. It’s perfectly possible. Wonder I hadn’t the wits to think of it for myself.’

‘So you see,’ said Poirot, ‘why I believe that the plans are still in the house. The problem now is to find them!’

Sir George snorted.