Emily is a normal teenage girl who is visited one night by an invisible entity that starts moving things around her room and attempting to get inside her. She pushes the demon off, but the demon -- or demons -- come back, and the next time they set up camp inside her, making life hell. In one particularly stark and memorable scene, she is sitting in a cafe and can hear nothing but the sound of forks and knives clinking against plates. The sense of brutal isolation is conveyed. Emily runs out onto the street, and as she passes ostensibly normal people, their faces transform into demon faces.

A minor metaphysical question: When Emily sees a demon in these people, is she hallucinating, or is she actually seeing what is really in people, even if they themselves are unaware? It's a chilling thought, just as chilling as the moment when Emily rushes into a church for sanctuary, looking as calm and unruffled as Munch's "The Scream," and finds that the people sitting in the pews have demon faces, too.

Laura Linney plays the priest's lawyer, Erin, a woman who has never given much thought to the spirit world. Her career is her religion. An agnostic, she has the task of trying to make the jury agnostic on the subject of science: A series of prosecution experts testify that Emily was suffering from an exotic form of psychosis with an epileptic component, and Erin's job is to make the jury doubt it. Curiously, and intentionally, "Emily Rose" has the scientists' explanation sounding more convoluted and far-fetched -- more voodoo, in its way -- than the priest's.

The continued presence of the spirit world unites the past and present halves of the movie, with invisible demons doing things to menace and unnerve Erin and the priest daily at 3 in the morning ("the witching hour"). Meanwhile, the viewer is never allowed to forget the visceral torment of possession. In "The Exorcist," Linda Blair was scary. In "Emily Rose," what's happening to Emily is scary. She tears her hair out, eats insects, experiences grotesque convulsions and scrapes her nails against the wall, but even as the demons are making her do these things, she remains in some way conscious and present. Talk about hell.

The film gives Linney a chance to play someone sharp and aggressive again after a few years of playing softies, lightweights, witnesses and victims. She expands in the role. Wilkinson lends intensity and conviction to the priest. As for newcomer Carpenter, one could easily overlook her merely as that poor possessed girl. But that's a performance, and a strong one.

The mood created by these human stars is augmented enormously by the film's other star -- the soundtrack -- which starts humming, vibrating and bringing on an ominous chorus every time it's 3 in the morning. The sound is loud, but the effect is so subtle that some people might not even notice it. It's the sound of being too scared to think straight.

"Exorcism" is being marketed as a horror film, but teens expecting midnight madness-style shocks are likely to be disappointed. Special effects scenes, including some that crib from the superior J-horror "The Ring," are passable but have been toned down to achieve the PG-13 rating. Horror buffs are likely to be put off by lengthy and wordy courtroom diatribes about the age-old battle between science and religion. The film's strong religious leanings could provide an additional marketing angle in some territories.

The story, by Derrickson and his regular writing partner, Paul Harris Boardman, is a standard riff on "The Exorcist" pegged to a trial. When Emily (Jennifer Carpenter) starts having fits and demonic hallucinations, she takes the recommended medicine from her doctor. When medicine seems to have little effect, she seeks the help of a Catholic priest, Father Richard Moore (Tom Wilkinson). The priest pronounces that Emily is possessed by the devil and advises her to stop taking her medication.

Emily subsequently dies during a violent exorcism. The priest is charged with negligent homicide. This story is pieced together in court by an agnostic defense attorney (Laura Linney) and contested at every step by a thoughtful prosecutor (Campbell Scott).

The bulk of the courtroom scenes center on a theological argument. Linney says that the jury should open their minds to the possibility of the existence of demons. Even if demons don't exist, the priest believes they do. This, she claims, absolves him of any moral crime, as he was acting in what he thought were Emily's best interests. Scott rebuts these two assertions and invokes good science to show that Emily was psychologically disturbed. There is no proof that supernatural beings exist, he says, and proof is what counts in court.

The filmmakers argue that these courtroom scenes show a balanced argument, but the structure of the film sabotages them. It's told from the point of view of the defense, so supernatural happenings are presented as concrete events.

Derrickson's characters are reduced to ciphers in a theological debate. Long wedges of the film are simply a discussion about the relative merits of science and superstition. Carpenter, as the sick girl, puts in the best performance, contorting her body into angular shapes during the exorcism sequences.