Seeking a fresh start for their marriage, British woman, Anna, and her Australian husband, Jake, move to Melbourne with their two small children. With Anna resuming her career, they hire a young live-in nanny to help.... but local girl Becky is anything but the innocent stranger she pretends to be. Noticing secretive behaviour in Jake, fragile Anna befriends Becky and reveals her suspicion that he's having an affair. Seemingly supportive, Becky encourages Anna to confront the woman, but Jake can explain everything, leaving Anna humiliated and vowing to never doubt him again. However, in a shocking twist, it's revealed that Jake and Becky are the ones having an affair, which began in London. As time passes, Becky begins to question her situation. Jake has convinced her that Anna is unstable, but is she truly - or is Jake making her that way? A cat and mouse game ensues, with Becky unsure of who to believe. Pushed to the brink, Anna realises she must turn the tables or she'll lose her kids, so she sets up an elaborate trap with deadly consequences.