Outside the supermarket with her young son, Cathy encounters Michael - the man convicted of the brutal murder of her brother, 20 years earlier. Ignoring the advice of her best friend, Cathy takes to social media, uncovering the successful career and family life Michael has forged for himself since early release from prison, while she grieved for her brother.

Re-traumatised by her past, Cathy instigates a clandestine relationship with the man she despises, embarking on a campaign of harassment and deceit. As Cathy's obsession grows, dark fantasies of revenge and reality blur as she sets out on a campaign of psychological warfare to destroy Michael's life.