It was seven in the evening of a lovely summer’s day, but Dr. Watson was sufficiently familiar with the irregularity of his old friend’s hours to feel no surprise at the idea.

“That means a case, I suppose?”

“Yes, sir, he is very hard at it just now. I’m frightened for his health. He gets paler and thinner, and he eats nothing. ‘When will you be pleased to dine, Mr. Holmes?’ Mrs. Hudson asked. ‘Seven-thirty, the day after tomorrow,’ said he. You know his way when he is keen on a case.”

“Yes, Billy, I know.”