A petty evasion of a minor chore - she sent her little brother to buy cigarettes for her dad - means a lifetime of bitter regret and the embrace of terrorism for young Colette McVeigh (Riseborough).
For on that fateful day, the little boy was fatally caught in the crossfire between Republican gunmen and the British forces patrolling the streets of 1970s Belfast.
Twenty years later she's in London to plant a bomb on the Tube...but British Intelligence are monitoring her every move. She's picked up...and presented with a stark choice - she either gets sent down for a very long time and her boy goes into care...or she works as an informer for MI5.
Clive Owen's Mac is her handler...and soon she's feeding back information that local IRA brigade leader is planning the assassination of the detective involved in the investigation of their little brother's death,
When the Provo hitman is taken out by an SAS bullet and Colette escapes, the witchhunt for the mole begins. However, when she turns to Mac for help it seems that his boss (Anderson) has an ulterior motive for Colette taking the heat.
Unfolding with a vice-like grip, this beautifully-paced thriller has echoes of Tinker Tailor Soldier Spy with its first-rate evocation of a nicotine-stained era where there was little place - on both sides - for honour or doing the right thing.
Riseborough's Colette is no malleable puppet - she has nerves of steel when facing off her IRA associates who suspect she's a turncoat - but is also a devoted mother whose heart you suspect was never into hardline violence.
A softly-spoken Owen convinces as the tough-but-decent Mac and Aiden Gillen puts in a spirited performance as Colette's brother, a Provo you rather think embodies the Never Surrender slogan.
A clever, intricate thriller with a killer twist.