In the year 1900, Sir Arthur Conan Doyle was at the height of his success as a doctor, a sportsman, a writer of historical novels, a champion of the oppressed and, most notably, the creator of that honorable, brave, and eminently sensible master detective, Sherlock Holmes. Every new Holmes story was greeted with great anticipation and confidence in the knowledge that, however complex the crime, the supremely intelligent and logical detective would solve it. But in 1916 Conan Doyle surprised his readers by declaring that he believed in Spiritualism. And when, in 1922, he published a book in which he professed to believe in fairies, his devotees were distinctly non-plussed. How could the man who invented the ultra-rational Holmes claim to believe in something as vague and unproven as the paranormal? Andrew Norman delves into both Doyle's medical records and his writings to unravel the mystery.