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Sophie Hartley (Sarandon) is convinced that she is being stalked. She becomes increasingly certain that her husband's beautiful co-worker, Mara (Blunt), wants her children, her husband and her life. But no one believes Sophie. Forced to prove her sanity, Sophie grows increasingly paranoid - but is she imagining things? Sophie becomes completely caught up in her obsession, turning stalker herself - and makes a discovery more frightening than her worst fear.