Wife and husband Paula and Jerome hire an au pair, Tess, to look after their baby, Orso, but she soon becomes an unsettling presence in their home.

Paula struggles to stay on course, and her marriage starts to feel the strain, as she discovers that Tess has been lying. Her psychiatrist suggests she may be suffering from postpartum depression.

Paula is arrested on suspicion of murder. Having discovered some long-held secrets, Jerome decides he wants a divorce, as Tess continues to weave her web in the family home.

Paula is on Tess's trail, locating the places where she grew up. She begins to realise that Tess is probably not who she says she is and that the family weren't targeted by chance.