"The Adventure of the Creeping Man" is a real head-scratcher of a Sherlock Holmes story -- not in the mystery itself, but in the way it will surely leave you asking: "what was Arthur Conan Doyle thinking?"
The personal secretary of a prestigious professor approaches Holmes. The professor has recently undergone an entire transformation of personality, in the wake of an engagement to a younger woman. He's altered his routine, is prone to violent temper fits, has been attacked twice by his own loyal dog, and has even been observed crawling around at night like some hunting creature. What can possibly explain this strange behavior?
Nothing, as it happens. Nothing realistic, anyway. I'll give the game away and reveal that Holmes discovers that the professor has been taking some drug in the hopes of recovering lost youth. Instead, it has worked a Jekyll-and-Hyde metamorphosis on him, all the way down to granting a few supernatural abilities and provoking the violent attack by the dog. None of this has any basis whatsoever in science, crossing well into the bounds of science fiction. It's a truly odd departure for Arthur Conan Doyle, to a degree where I'd actually doubt his authorship of the story were the history and the scholars not otherwise certain on the matter.
The result is a truly disappointing tale. It's not badly written as such, but it feels as though the long-standing contract with the readers (of just what Sherlock Holmes is about) has been violated. It's like a Scooby-Doo episode in which the ghost at the end is real and not some guy in a rubber mask. Especially peculiar is the way Holmes berates himself near the end of the tale for not having deduced the solution to this mystery sooner -- as though he somehow should have known that a drug exists that can transform a man into some kind of pseudo-lycanthrope monster.