Dr. Miranda Gray (HALLE BERRY) is a staff psychologist at the Woodward Penitentiary for women where she works with inmates such as Chloe Sava (PENELOPE CRUZ) who murdered her stepfather but claims to be raped by the Devil.
After saying goodbye to her husband and boss, Doug Grey (CHARLES DUTTON), and being escorted to her car by coworker Dr. Pete Graham (ROBERT DOWNEY, JR.), Miranda learns from Sheriff Bob Ryan (JOHN CARROLL LYNCH) that the road ahead of her is closed.
Accordingly, she takes the detour, but after crossing a bridge at night in the middle of a bad storm, she nearly hits a dazed-looking teen standing in the middle of the road. When she goes to check on the girl, the ghostly looking figure suddenly bursts into flames, as does Miranda.
When she wakes up, three days have passed and she's an incarcerated inmate at the same mental ward where she works. Accused of brutally murdering her husband, Miranda has no memory of what transpired after meeting the ghostly bridge girl. She then finds herself locked up with her former patients, including Chloe, who tells her no one will believe what she says since everyone now thinks she's crazy.
Things only get worse when the ghostly bridge girl starts showing up with the message "Not Alone," and Pete, who's treating Miranda, really begins to doubt her sanity. From that point on, and as she begins to suspect all of this had something to do with the late daughter of fellow doctor Phil Parsons (BERNARD HILL), Miranda sets out to do whatever it takes to prove her sanity and get to the bottom of her husband's murder and the ghostly occurrences.