One day in 2009, twenty-four-year-old Susannah Cahalan woke up alone in a strange hospital room, strapped to her bed, under guard, and unable to move or speak. A wristband marked her as a "flight risk," and her medical records, chronicling a month-long hospital stay of which she had no memory at all, showed hallucinations, violence, and dangerous instability. Only weeks earlier, Susannah had been on the threshold of a new, adult life: a healthy, ambitious college grad a few months into her first serious relationship and a promising career as a cub reporter at a major New York newspaper. Who was the stranger who had taken over her body? What was happening to her mind? In this swift and breathtaking narrative, Susannah tells the astonishing true story of her inexplicable descent into madness and the brilliant, lifesaving diagnosis that nearly didn't happen. A team of doctors would spend a month, and more than a million dollars, trying desperately to pin down a medical explanation for what had gone wrong. Meanwhile, as the days passed and her family, boyfriend, and friends helplessly stood watch by her bed, she began to move inexorably through psychosis into catatonia and, ultimately, toward death. Yet even as this period nearly tore her family apart, it offered an extraordinary testament to their faith in Susannah and their refusal to let her go. Then, at the last minute, celebrated neurologist Souhel Najjar joined her team and, with the help of a lucky, ingenious test, saved her life. He recognized the symptoms of a newly discovered autoimmune disorder in which the body attacks the brain, a disease now thought to be tied to both schizophrenia and autism, and perhaps the root of "demonic possessions" throughout history.