Hanslet, whose memory for names and facts was rarely at fault, recounted as briefly as he could the course of events from the murder of Mr. Tovey in November, to the finding of Martin’s body in the cellar of Number 407, in January. The Professor interrupted him now and then to ask a question, but in the main he allowed him to tell the story in his own way. When he had finished, and the Professor had expressed himself satisfied, Hanslet continued.

“The man who’s been in charge of the case is a fellow called Whyland, keen enough on his job, but a bit lacking in imagination. I had a chat with him yesterday, and he confessed that he was completely at the end of his tether. Up till last Saturday evening, he told me, he was pretty sure that he could lay his hand on the criminal, but that night something happened which entirely upset his calculations.”

“What was that?” enquired the Professor, who was listening intently.