As her 40th birthday approaches Emma stops sleeping. She realises she is repeating the same unsettling behaviour as her mother, who had a psychotic break on her own 40th. Is she turning into the very monster she has spent a lifetime trying to forget?

After their mother unexpectedly dies, Emma and Phoebe are shocked to learn her death is suspicious. Reeling from recent events, and with worsening insomnia, Emma is forced to come clean to her husband Robert about her past.

As Emma's life continues to unravel, her unsettling nighttime behaviour escalates so she decides to investigate what's happening to her. It becomes apparent that Emma may pose a danger to her own family which has devastating consequences.

Emma seeks solace in a new friend while Phoebe replaces her sister in the family home. As Emma digs deeper into the past, a forgotten childhood memory shines a light on her relationship with Phoebe, causing her to doubt her sister's intentions.

As Emma comes under the spotlight, she becomes increasingly convinced of her sister's treachery. Before she can get answers to a newly emerging memory, tragedy strikes and Emma begins to mistrust herself.