Sophie Hartley (Susan Sarandon) is convinced that she is being stalked. She becomes increasingly certain that her husband's (Sam Neill) beautiful co-worker, Mara (Emily 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.