There was complete silence for a minute and a half. Then I laughed.

'You're mad,' I said.

'No,' said Poirot. 'I am not mad. It was the little discrepancy in time that first brought my attention to you - right at the beginning. You left the house at ten minutes to nine - both by your own statement and that of Parker, and yet it was nine o'clock when you passed through the lodge gates. It was a cold night - not an evening a man would want to walk slowly; why had you taken ten minutes to do a five minutes' walk? All along I realized that we had only your statement that the study window was closed. Ackroyd asked you if you had done so - he never looked to see. Supposing, then, that the study window was open? Supposing, too, that you killed Ackroyd before you left? Then you go out of the front door and run round to the summerhouse. You then take Ralph Paton's shoes out of your black bag, put them on, walk through the mud in them, and leave prints on the windowsill when you climb in. You lock the study door on the inside and run back to the summerhouse. Then you change back into your own shoes, and run down to the gate. I did exactly the same things myself the other day - it took ten minutes exactly. Then you went home, which gave you your alibi - since you had timed the Dictaphone for half-past nine.'

'My dear Poirot,' I said, 'what on earth had I to gain by murdering Ackroyd?'

'Safety. It was you who was blackmailing Mrs Ferrars. Who knew more about Mr Ferrars' death than his doctor? When you spoke to me that first day in the garden, you mentioned a legacy that you received about a year ago. I have been unable to discover any trace of a legacy. You had to invent some way to explain the twenty thousand pounds you blackmailed out of Mrs Ferrars - money that has not done you much good. You lost most of it in speculation - and then you went back to Mrs Ferrars for more money - and Mrs Ferrars took an unexpected way out of the torture you were putting her through. You knew that if Ackroyd learnt the truth, he would have no mercy on you - you were ruined.'

'And the telephone call?' I asked. 'You have an explanation of that also, I suppose?'