It stars David Caruso, Peter Mullan, Brendan Sexton III, Josh Lucas, and Gevedon as an asbestos abatement crew who take a clean-up job at an abandoned mental asylum amid an intense work schedule, growing tensions, and mysterious events occurring around them.

His crew includes Mike, a law school dropout who is knowledgeable about the asylum's history; Phil, who is dealing with his grief over a recent breakup; Hank, a gambling addict; and Gordon's nephew Jeff, who has a pathological fear of the dark.

The men begin their job, and Mike discovers a box containing nine audio-taped therapy sessions that were recorded with Mary Hobbes, a patient who suffered from dissociative identity disorder.

In the sessions, Mary's psychologist attempts to unveil details surrounding a crime she committed at her home two decades prior.

Gordon confides in Phil that he slapped his wife Wendy after she inadvertently splashed him with boiling water, and that she refuses to answer his calls or let him see their infant daughter.

Mike restores the electricity and continues listening to the ninth and final tape, which reveals that one of Mary's malignant personalities, "Simon", was responsible for her murdering her family.

Phil finds Gordon in Mary's former hospital room, staring at photos from his daughter's baptism which he has pasted to the wall.

The following day, Gordon arrives at the hospital and finds Hank wrapped in plastic sheeting in one of the rooms, the lobotomy pick protruding from his eye.

In reviewing the film for the 2003 edition of The Year's Best Fantasy and Horror, Edward Bryant contends that Simon is not necessarily an alternate personality of the former patient Mary, but rather a malevolent genius loci.

"[9] The film's plot was inspired by the Richard Rosenthal case, a murder that took place in Boston, where Anderson grew up, in the mid-1990s, in which a man supposedly killed his wife after she accidentally burnt his dinner, then cut out her heart and lungs and put them in his backyard on a stake.

Don't Look Now, directed by Nicolas Roeg, was one inspiration for the film, for its sense of place and because the lead character realizes in the climax that he is at the heart of the mystery.

[9] Most of the film was shot in a small section of the Danvers Asylum;[12] according to actor David Caruso, the rest of the building was "unsafe" for shooting.

[19] Los Angeles Times said of the film: "Session 9 is so effective that its sense of uncertainty lingers long after the theater lights have gone up.

A negative review came from Variety, which wrote, "while pic works up a nervously eerie paranoia, it finally doesn't know what to do with what it sets up.

"[23] San Francisco Chronicle said, "the story doesn't quite pay off, characters are underwritten and the surprise ending is contrived and unconvincing.

"[24] The Village Voice wrote, "the script for Session 9 is so underwritten that even such lively character actors as David Caruso, Peter Mullan and Brendan Sexton III are left stranded.

"[25] Dave Kehr, in a mixed review for The New York Times, praises the "impeccable" performances and the dialogue's "authentic working-class snap", but criticizes the pacing which "often feels long and aimless", and concludes that the film "loses any sense of urgency or structure" because of Anderson's choice to leave the connections between events unstated.