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.