Psychologist Oscar Loste is facing the most confusing and frightening case of his career. Six-year-old Beth Bardo is displaying bizarre behavior, including interaction with an imaginary friend she calls Mister Smiler. This invented character supplies Beth with an impossible degree of knowledge about everything from science to ancient cultures to Oscar’s own secrets. The little girl’s parents, a highly-educated couple who also happen to be one of the wealthiest in the country, seem as bewildered as Oscar over Beth. The more he gets to know the little girl and her family, the more Oscar slips into chaos. Self medication with a mysterious cough syrup recently taken off the market is plaguing him with strange hallucinations and fueling his obsession with the case. As the chilling truth about Beth and Mister Smiler unfolds during her therapy sessions, Oscar must face consequences beyond his imagination.