Theresa sat down on her favourite low stool and looked impatiently at Poirot.
“Well, you’ve seen Purvis? What about it?”
Poirot said in a noncommittal voice:
“There are—possibilities, mademoiselle.”
She looked at him thoughtfully. Then she sent a very faint glance in the direction of the doctor. It was, I think, intended as a warning to Poirot.
“But it would be well, I think,” went on Poirot, “for me to report later when my plans are more advanced.”
A faint smile showed for a minute on Theresa’s face.
Poirot continued:
“I have today come from Market Basing and while there I have talked to Miss Lawson. Tell me, mademoiselle, did you on the night of April 13th (that was the night of the Easter Bank Holiday) kneel upon the stairs after everyone had gone to bed?”
“My dear Hercule Poirot, what an extraordinary question. Why should I?”
“The question, mademoiselle, is not why you should, but whether you did.”
“I’m sure I don’t know. I should think it most unlikely.”
“You comprehend, mademoiselle, Miss Lawson says you did.”
Theresa shrugged her attractive shoulders.
“Does it matter?”
“It matters very much.”
She stared at him. In a perfectly amiable fashion, Poirot stared back.
“Loopy!” said Theresa.
“Pardon?”
“Definitely loopy!” said Theresa. “Don’t you think so, Rex?”
Dr. Donaldson coughed.
“Excuse me, M. Poirot, but what is the point of the question?”
My friend spread out his hands.
“It is most simple! Someone drove in a nail in a convenient position at the head of the stairs. The nail was just touched with brown varnish to match the skirting board.”
“Is this a new kind of witchcraft?” asked Theresa.
“No, mademoiselle, it is much more homely and simple than that. On the following evening, the Tuesday, someone attached a string of thread from the nail to the balusters with the result that when Miss Arundell came out of her room she caught her foot in it and went headlong down the stairs.”
Theresa drew in her breath sharply.
“That was Bob’s ball!”
“Pardon, it was not.”
There was a pause. It was broken by Donaldson who said in his quiet, precise voice: