After learning the police investigation into the fatal hit and run of her young son Martie has been dropped, Frances Cairnes (Cush Jumbo) takes matters into her own hands. Posing as a novelist researching a new murder-mystery, Frances ingratiates herself with the family of George Rattery (Jared Harris), the man she suspects is responsible, and sets a plan in action to kill him. She's tracked by detective Nigel Strangeways (Billy Howle), who has reason to believe his department covered up George's involvement in the incident. Suffering from PTSD following the recent death of his partner, Strangeways throws himself into pursuing justice for Martie. But when he discovers an "undercover" Frances living with George and his family, Strangeways finds himself working to both prove George's guilt and head off Frances's plans for revenge before anyone else dies.