In 2003, Detective Inspector Sarah Clayton (
Joanne Froggatt) tries to establish why British schoolboy Mark (
Jamie Blackley) would want to stab John (
Toby Regbo). The two were supposed to be friends, yet Mark claims that the crime was necessary. As she looks deeper into the crime and the two teenagers, she discovers that Mark frequented chatrooms and became fascinated with Rachel (
Jaime Winstone), a woman he met in one such room. With her prompting, he agrees to make friends with her brother John. Mark quickly begins to fall for her, sympathising with her tales of
domestic abuse by her boyfriend Kevin (Mingus Johnston) while believing that they cannot meet because she is in a
witness protection program. Mark is horrified when John informs him that Kevin has killed Rachel, and he begins planning revenge against Kevin. Mark's sanity has been slowly coming apart, culminating in his beginning his plans to stab John after a chat with Janet (
Liz White), an
MI5 agent, who tells him that John is a
person of interest to the agency. It is eventually revealed that John
orchestrated the entire plot and that there is no Rachel or Janet, and that he pretended to be various people in order to get to Mark. ==Cast==