So our enemy agent, murderer or not, looked like Douglas Grace or Fabian Losse. Both had free access to the stuff and the means of passing it on. My job was to find the agent. As a working proposition I supposed he was also the murderer. If, then, the murderer was the agent, the two likeliest bets for the agent were Losse and Grace. Which of them shaped up best as the murderer? Grace. Grace put the coat with the diamond clips over Mrs Rubrick’s shoulders and leant over her chair on the tennis lawn. Grace, therefore, could most easily remove one of the clips. It was obvious that the thing would have been found if it had lain glittering on bare earth under a scraggy zinnia. I tried it and it showed up like a lighthouse.

Grace egged on his aunt to organize a search-party and take herself off to the wool-shed. I wondered if he’d pinched the clip in order to bring about this situation and dropped it among the zinnias for Arthur Rubrick to find when Grace himself had finished the job in the wool-shed. If so, he was a quick thinker and a bold customer altogether. He’d hatched the whole project between the time Mrs Rubrick said she was going to the wool-shed and the moment when she actually left.

Grace had gone up to the house during the search and had answered the telephone and fetched two torches. This would give him a chance to bolt out by the dining-room windows and up the track to the wool-shed. On his return he could hang out the placard on Mrs Rubrick’s bedroom door. This placard is important. You’ll have noticed that he was the only member of the party who had the opportunity to do this.

You’ll notice too, that the disposal of the body must, unless our expert was Markins, Albert Black, or the quite impossible Mrs Duck, have been interrupted by a period, not shorter than the interval between the end of the search and the going to bed of the household and the cottage; and not longer than that between the assault and the onset of rigor mortis. Now, the abominable Black gouged out the candle stump which had been pressed down, almost certainly by the murderer, and chucked it into the pens. If he were guilty he would hardly have volunteered this information.

But suppose Grace was our man?