Session 9 is the first film released from director Brad Anderson after his lovely 1998 breakthrough, the bossa nova sparked ''Next Stop Wonderland,'' and this movie is so different in tone that the filmmaker, at times, seems to be bending over backward to stretch his wings. Anderson stumbled upon the Danvers Mental Hospital, an ornately oversize Massachusetts complex that had been closed down since 1985, and discovered a high ceilinged maze of dusty, crumbly chambers, the peeling paint catacombs still crammed with ominous medical paraphernalia: examination tables, steel hydrotherapy baths. He must have realized that this could be as organic a setting for a new style daylight gothic as the Overlook Hotel in ''The Shining'' or the deranged Danish hospital of Lars von Trier's ''The Kingdom.''
Shot on HD video, ''Session 9'' is a marvel of vérité nightmare atmosphere. It follows five asbestos removal workers, led by the solicitous yet troubled Scottish emigrant Gordon (Peter Mullan), as they spend a week clearing the rooms of poison panels (the place is so huge that it looks as if they would actually need three years). At moments, you can just about smell the invisible fibers, and the ghosts of former patients as well. This is a movie in which the 1970s and '80s loom as the graveyard of the psychoanalytic past, and there are tantalizing references to multiple personality disorder and repressed memory syndrome.
The trouble is, they remain motifs, scripted rather than connected. The story, a potpourri of fright devices (midnight stalker, overly foreshadowed lobotomy maiming), keeps undercutting the grainy ''Blair Witch'' mood established by Anderson's intuitive camera eye. If ''Session 9'' proves anything, it's that what today's audiences may have to fear more than the return of the repressed is the return of repressed horror film conventions.