Ellen gets pregnant after landing a big contract for her firm of architects. She hires Paula as cover but soon fears she's made a terrible mistake. Ellen wants to get back to work quickly but Paula thinks Ellen should be putting her baby at the centre of her life. She wastes no time in moving in on Ellen's bosses, clients and friends and Ellen fears she's being forced out. Everyone thinks Paula is great and Ellen is being paranoid until a site accident puts Ellen's suspicions into overdrive.
Ellen tries to juggle her new baby with her return to work and her investigation into Paula. She finds evidence that her boss was having an affair but her only lead is Paula's ten-year-old child and a former colleague of Paula's, neither of whom want to speak to her. Paula discovers Ellen's investigation and engineers a humiliation from the client and a warning visit from social services. Ellen eventually confronts Paula and forces her to confess what she's been hiding.
After Paula confesses her secret, Ellen thinks she has proof that Paula is disturbed but no one else sees it her way. The office turns against Ellen and she is forced to leave for good. She tries to put Paula behind her but Paula isn't willing to let go. When Ellen encounters Paula's mysterious ex-colleague at an interview, Paula retaliates by driving a wedge between Ellen and her husband. After a catastrophic celebration at Ellen's finished building, her family as well as her career face ruin.