Sophie Hartley (Susan Sarandon) is a successful illustrator, a dedicated mother and a loving wife, but things aren't going well with her husband, Craig (Sam Neill). Sophie is convinced that she's being stalked by one of Craig's co-workers - his beautiful assistant, Mara Toufiey (Emily Blunt) - but nobody believes her. Sophie's life begins to fall apart, as she feels she's being watched, followed, even copied. She fends off paranoia, but it is clear to her that Mara is trying to steal her husband, her daughters and her life... Now Sophie is forced to become the stalker, but when she crosses the boundaries of acceptable behaviour, Mara takes out a court order against her. She begins to fight back, but before she attempts to regain the trust of her family, she discovers something more frightening than anything she has ever encountered - a dark secret from the past, that will change her life forever.