The lives of the Barretts, a normal suburban New England family, are torn apart when fourteen-year-old Marjorie begins to display signs of acute schizophrenia.

To her parents' despair, the doctors are unable to stop Marjorie's descent into madness. As their stable home devolves into a house of horrors, they reluctantly turn to a local Catholic priest for help. Father Wanderly suggests an exorcism; he believes the vulnerable teenager is the victim of demonic possession. He also contacts a production company that is eager to document the Barretts' plight. With John, Marjorie's father, out of work for more than a year and the medical bills looming, the family agrees to be filmed, and soon find themselves the unwitting stars of The Possession, a hit reality television show. When events in the Barrett household explode in tragedy, the show and the shocking incidents it captures become the stuff of urban legend.

Fifteen years later, a bestselling writer interviews Marjorie's younger sister, Merry. As she recalls those long ago events that took place when she was just eight years old, long-buried secrets and painful memories that clash with what was broadcast on television begin to surface - and a mind-bending tale of psychological horror is unleashed, raising vexing questions about memory and reality, science and religion, and the very nature of evil.