Johnny Worricker (Bill Nighy) is a long-serving MI5 officer. His boss and best friend Benedict Baron (Michael Gambon) dies suddenly, leaving behind a contentious file whose explosive contents threaten the stability of the organisation. Meanwhile, a seemingly chance encounter with Johnny's striking next-door neighbour and political activist Nancy Pierpan (Rachel Weisz) seems too good to be true. Johnny is forced to walk out of his job, and then out of his identity to find the truth.