After the death of her husband, Dr. Cara Harding's (Julianne Moore) faith in God has been shaken, but not her belief in science. In an attempt to open her up to accepting unexplainable psychiatric theories, her father introduces her to Adam, a patient with multiple personalities who takes on some of the physical characteristics of his other personalities. But, Cara quickly discovers that Adam's other personalities are murder victims and the more she finds out about him and his past, the closer she and her loved ones are to becoming murder victims themselves.