Opening on a beautiful stretch of coastland somewhere in Canada, a dishevelled looking 30-year-old Martin McGartland (Jim Sturgess) walks towards his car and checks under the wheels for anything suspicious. He gets in and starts the engine. It appears the routine is going to plan until a gunman appears at the window and fires six bullets into Martin leaving him dying and covered in blood.

A voiceover from Fergus (Ben Kingsley), a stern sounding older British police officer, introduces us to Martin who was an informant for the British Special Branch during the war in Northern Ireland. The story flashes back 11 years to Belfast 1988: Martin is a cheeky 20-year-old street hustler flogging garments door to door. Martin and his mate Sean (Kevin Zegers) try to survive in the terribly repressed Catholic community of West Belfast. British police patrols regularly stop and search young Catholics while the IRA punishment squads rule the estates with threats of knee-capping – shooting bullets through the back of knees.

Martin and Sean escape a police check and sprint away through the back alleys of their community. Martin is captured by the Police and is asked if he would like to help as an informant but Martin does not want to grass on his own community. Fergus watches with interest believing Martin has a mind of his own that could be useful on the streets of West Belfast.

Martin is released and meets up again with Sean at Paddy’s warehouse where Sean introduces Martin to an IRA operative called Ray. Martin is a bit concerned Sean and Paddy are so close to an IRA operative. Sean is clearly more enamoured by the life the IRA can offer.

One night Martin sees a teenage lad, Frankie, in the neighbourhood being knee-capped for stealing and he tries to intervene. Ray is the hitter and removes his hood and drags Martin off to meet his superior Micky Adams. Micky acknowledges Martin can’t have grassed on him when taken in by the police and offers Martin a job driving for him. Martin accepts.

Fergus hauls Martin in for interrogation again and wants to offer Martin a car and some cash to provide low level

information on any IRA activity. In spite of what has happened to Frankie, Martin isn’t keen to help ‘the peelers’ but he agrees to take the car and Fergus’ number and code name.

Martin goes to visit Frankie with his new car and meets Frankie’s sister, Lara (Natalie Press). He invites her out on a date.

On one of Martin’s first missions for Micky Adams he drives to a hotel out of town and one of his passengers gets out to deliver a package. Martin does not think much of it and takes Lara for a night out with his new earnings. They hit it off and have sex on the rooftop of the Europa Hotel but on their return they witness the aftermath of a massive bomb at the hotel where Martin had driven to earlier on with the passenger and the package. Many soldiers and civilians are killed. Martin is suddenly horribly compromised and tries to call Fergus. He is interrupted but knows that he must try to help stop the killings.