The Adventure of the Creeping Man by Arthur Conan Doyle

A professor begins acting bizarrely—climbing walls and behaving like an animal—and Holmes is asked to determine if madness or something more sinister is at play. Holmes discovers that the man has been injecting himself with a serum derived from apes in an attempt to regain youth for a younger lover. The plot combines Gothic horror with speculative science. It’s one of the strangest Holmes tales, exploring vanity, aging, and the consequences of tampering with nature. A bizarre yet compelling cautionary mystery.