Content Warning: This section contains detailed accounts of crime, including sexual assault; refers to attempted death by suicide; and discusses trauma and abuse as well as institutionalization and mental health treatment.

Author Daniel Keyes explains that the book is a factual account (through 1981) of the life of William Stanley “Billy” Milligan, the first person in US history found not guilty of a major crime by reason of insanity due to a diagnosis of “multiple personality disorder,” now known as dissociative identity disorder (DID). Keyes collected information from Milligan primarily by interacting with the “fused Billy,” an amalgamation of the several alters Milligan displayed, though he interacted with several of Billy’s alternate identities (alters) as well. In addition, Keyes spoke with 62 people who knew and engaged with Milligan at different points in his life. Most people who interacted with Milligan came to accept his diagnosis, while some remained skeptical and considered him a fraud. Keyes interacted with people in both camps, but over the course of the two years he worked with Milligan on the book, his own skepticism turned to belief.