Emma is a teenager who is having problems. Right after she goes into a seizure her parents take her to the hospital, where they do numerous tests. Everything is negative. But she soon comes to believe that she's possessed by an evil spirit and asks her uncle's help. She blanks out and does horrible things, like nearly drowning her younger brother in bathtub and causing a potential boyfriend to get in a fatal car crash. The parents are not religious and are reluctant to have the uncle do the procedure but ultimately agree after they witness her levitating off the ground. He does this in their home and videotapes the sessions, in which her eyes turn white and she speaks in a different voice. It's revealed that he had tried doing an exorcism before--but the young girl died as a result, and we get the sense that he wants to redeem himself and do it right this time.
What really makes this movie is the "twist", which makes complete sense and adds an entirely different dimension to the film. Doug Bradley (Pinhead from the Hellraiser movies) has a brief role as another exorcist. Highly recommended. Next to the original Exorcist movie, this is one of the best possession movies I've seen.