‘And I,’ Troy said, ‘was putting the trap away and coming in by the east wing door. If…Suppose I’d let Sonia do that and taken the medicine into the school—’

‘If you’ll excuse my interrupting you, Mrs Alleyn,’ Fox said, ‘it’s our experience that, when a woman makes up her mind to turn poisoner, nothing will stop her.’

‘He’s right, Troy.’

‘Well,’ said Troy, ‘go on.’

‘She had to chip away the chemist’s sealing-wax before she got the corks out, and Fox found bits of it on the floor and some burnt matches. She had to find another bottle for her purpose. She emptied Sir Henry’s bottle, filled it up with thallium, and in case of failure, poured the remainder into her own small phial. Then she filled the children’s bottle with water and re-corked and re-sealed both Mr Juniper’s bottles. When Miss Able came in for the children’s medicine she and Millamant hunted everywhere for it. It was not found until Fenella came downstairs, and who was more astonished than Millamant to learn that Sonia had so carelessly left the medicine in the flower-room?’

‘But suppose,’ Troy said, ‘he’d wanted the medicine before she knew about the Will?’

‘There was the old bottle with a dose still in it. I fancy she removed that one some time during the Birthday. If a Will unfavourable to Cedric had been made public, that bottle would have been replaced and the other kept for a more propitious occasion. As it was, she saw to it that she was never alone from dinner until the next morning. Barker beat on the door of the room she shared with Desdemona. She had talked to Desdemona, you remember, until three o’clock—well after the time of Sir Henry’s death. She built herself up a sort of emergency alibi with the same elaborate attention which she gives to that aimless embroidery of hers. In a way this led to her downfall. If she’d risked a solitary trip along those passages that night to Cedric’s room she would have heard, no doubt, that Sir Henry had signed the second Will, and she would have made a desperate attempt to stop him taking his medicine.’

‘Then she didn’t mean at that time to throw suspicion on Sonia?’

‘No, indeed. His death would appear to be the natural result of rash eating and pure temper. It was only when the terms of his last Will were made known that she got her second idea.’

‘An atrocious idea.’