Rahn takes readers behind the fiction to explore the world from which Sherlock Holmes emerged. The book discusses the origins of forensic techniques such as fingerprinting, blood testing, crime-scene photography, and footprint analysis, while comparing these methods with Holmes's famous powers of observation and deduction. It also examines the literary influences behind Holmes, including Edgar Allan Poe's pioneering detective stories, and provides a fascinating portrait of crime, policing, and scientific investigation in Victorian London.