“If, then, Mortimer Tregennis disappeared from the scene, and yet some outside person affected the cardplayers, how can we reconstruct that person, and how was such an impression of horror conveyed? Mrs. Porter may be eliminated. She is evidently harmless. Is there any evidence that someone crept up to the garden window and in some manner produced so terrific an effect that he drove those who saw it out of their senses? The only suggestion in this direction comes from Mortimer Tregennis himself, who says that his brother spoke about some movement in the garden. That is certainly remarkable, as the night was rainy, cloudy, and dark. Anyone who had the design to alarm these people would be compelled to place his very face against the glass before he could be seen. There is a three-foot flower-border outside this window, but no indication of a footmark. It is difficult to imagine, then, how an outsider could have made so terrible an impression upon the company, nor have we found any possible motive for so strange and elaborate an attempt. You perceive our difficulties, Watson?”

“They are only too clear,” I answered with conviction.