The original drawing is available (ink & watercolour, 5x7 inches).
There's nothing to 'get' about this one. No jokes, no message. I really just wanted to draw an older character, and I have to admit I also enjoy a good Agatha Christie mystery, like William here. In fact, the language in her books has inspired a number of drawings.
I read somewhere that Edward Gorey was a huge Agatha Christie fan, and some of his stories have lines and characters that seem plucked from one of her crime scenes.