What's more important: loyalty to one's family or to a cause? That's the conundrum for Collette McVeigh (Andrea Riseborough), an Irish Republican from Belfast who's just been nicked by British intelligence for trying to plant a bomb in the London Underground. But she's a single mom with a young son, so an MI5 agent named Mac (Clive Owen) tells her she can either cooperate and become an informant or go to jail for so long that her boy will grow up without her. Collette lives with her mother (Brid Brennan) and two brothers, Gerry (Aidan Gillen) and Connor (Domhnall Gleeson), both of whom shoulder considerable responsibilities for the Irish Republican Army. There was another McVeigh sibling, but when the film opens in the 1970s, a young Collette deputizes her little brother Sean to go out for their father's cigarettes, and he dies after being caught in the cross fire between British and Irish forces. Fast-forward to 1993: The ostensible end to the Troubles is still a few years off, but it's obvious the childhood trauma has driven a guilt-ridden Collette to sympathize with the fight to force the British out by any means necessary. Although reluctant at first, Collette eventually consents to Mac's terms, and they agree to meet in secret on the Belfast coast (the film was shot in Dublin), where she's expected to report on IRA activities in general and her brothers' participation in particular. The latter proves to be a dangerous test of her loyalty after Connor ropes her into an assassination attempt on a police detective, but when it's thwarted by authorities, a relentless IRA security chief (David Wilmot) smells a rat -- and wonders if it's someone in the McVeigh family. The film was adapted by Tom Bradby from his 2001 novel, which was hatched during his time as a political reporter in Northern Ireland in the 1990s for Britain's Independent Television News. One day, Bradby spent his lunch hour talking to a male British intelligence officer about the business of recruiting agents in the IRA, and that afternoon being wowed at a press conference by a stunning Republican woman, causing him to wonder what would happen when these two worlds collided. Owen is the big draw here as Mac, who at first appears as if he's only here to pry the truth from Collette, but becomes more complex when it's revealed that his own colleagues, and especially his boss (Gillian Anderson), have been keeping him in the dark about an ongoing parallel operation. But Riseborough comes away as perhaps the best reason to see the film, as her guarded delivery, defiant stares, and the way she masks her constant fear of being found out fit neatly with the steadily mounting suspense, numerous close calls, and a long list of characters who may or may not be on the level. Director James Marsh won an Oscar for his 2008 documentary Man on Wire, about daredevil Philippe Petit's tightrope walk between the Twin Towers of New York's World Trade Center in 1974, but he ramps up a different kind of tension with Bradby's screenplay. And by keeping politics in the background and emphasizing more than a few tenuous relationships, he allows the conflict, mistrust, and dishonesty in both Collette's and Mac's camps to come through without taking sides.