My mom had an acting job up in DC and took me with her for the duration of the shoot. As a young man I was quite curious and after playing with an Ouija board I began to act strangely such as making mysterious noises, constant bad language and abnormal strength. I also caused my bed to shake as well, much to my mom's surprise. Despite giving a few unpleasant operations, Dr. Klein (a family friend) and his assistants find nothing wrong with me, not knowing I am now possessed by Pazuzu.
Then, one night I killed the babysitter and My mom's director, Burke Dennings, who My mom had a crush on (as she and my dad had divorced). His murder is investigated by a detective named William Kinderman. He interviews both a young priest named Damien Karras, who has lost faith in God after his frail mother died, and Mom. Mom knows I was the only one who was in the house when Dennings died.
After the doctors decide that an exorcism may be the only way to help me, Mom arranges a meeting with Karras. Karras at first refuses to get permission to perform an exorsism, despite me now being totally possessed by the demon. After getting a recording of me talking in reverse and seeing the words "Help Me" on my belly Karras decides to perform an exorcism with Merrin chosen to help.
In my bedroom both men try to exorcise the demon but a stubborn Pazuzu toys with them, especially Karras, including insulting his late mother. After a break and hint of faltering, Karras is dismissed. Merrin attempts the exorcism alone. Karras enters the room and discovers Merrin has died and confronts the mocking, laughing spirit of Pazuzu. At Karras' plea, Pazuzu then possesses Karras, leaving my body. In a moment of self-sacrifice, the priest throws himself out of the window and dies of his wounds, thereby completing the exorcism of Pazuzu.
A few days later I, now back to normal, return home to Los Angeles with my mother. Kinderman, who narrowly misses them, befriends Father Dyer, an old friend of Karras, as he investigates how Karras died.