Herman Webster Mudgett built a life on aliases, insurance fraud, and eventually murder, becoming known to history as H.H. Holmes. This film traces his path from a small-town upbringing through medical school to Chicago, where he constructed the building later nicknamed the Murder Castle, a maze of windowless rooms, gas lines, and trapdoors he designed himself. Set against the backdrop of the 1893 World's Fair, when the city was flooded with visitors and easy for people to disappear from, the documentary lays out how Holmes lured victims with promises of jobs or marriage before killing them for insurance money or, investigators believed, for the act itself. It follows the fraud schemes that first drew police attention, the confession that put a number on his victims, and the trial and execution that ended his run. Interviews and narration walk through the physical evidence found in the building after his arrest, piecing together how one man operated undetected for years in a city already straining to police itself.