Ex-policeman Luke Fitzwilliam decided that he was delighted to be home, able to sit back and relax after a life time's hard work abroad. He surveyed the railway carriage and decided that the nice old lady opposite wasn't going to travel in silence for long. He was right. Miss Pinkerton, obviously an old hand, had him deep in conversation in no time at all.
"... You were in the police? Now that really is interesting. In fact, quite a coincidence! Because you see, I am on my way to Scotland Yard!"
"Really? Why?" said Luke.
"Well, you see, our village constable is a dear fellow, but I don't think he's quite the person to deal with murder."
Luke's eyebrows rose. "Are you sure you mean murder? Couldn't you have been mistaken?" he asked gently. "I might have been the first time, but not he third, or the fourth. I mean one knows, doesn't one?"
Later, after the old lady died, Luke felt bound to take over where Miss Pinkerton had unwillingly-left off...