An active member of the IRA (Andrea Riseborough) is arrested during a failed bomb plot in London Underground. The woman agreed to become an informant for the service MI5 (represented by Clive Owen and Gillian Anderson) for the sake of his minor child. She turns in Belfast, his family is fully involved in IRA operations with lesser or greater violence.
The story is complicated since the first mission. The woman is pressured by authorities to deliver information on IRA operations and “brothers” of a network suspected of treason and it risks death. In the other camp, which recruits agent (Clive Owen) is responsible for her life, but gets tangled up in games and scenes from MI5 that can lead to the killing of its source.