Long before psychiatry existed, unexplained shifts in a person's behavior were blamed on possession by evil spirits, and this film traces how that belief hardened into a medical diagnosis. It follows the story from ancient accounts of possession through the 1800s, when doctors began treating dissociative behavior as a condition called hysteria rather than a spiritual affliction. Case histories and period explanations show how clinicians of the era tried to categorize patients who seemed to switch between distinct identities, and how those early theories fed into what would later become the modern understanding of dissociative identity disorder. The film stays close to the historical record, using period terminology and case descriptions rather than dramatization, and treats the shift from supernatural explanation to clinical diagnosis as its central thread. It is a compact look at how one of psychiatry's strangest and most disputed diagnoses got its start.