Traumatised ex-squaddie Robert Miller (Kebbell) finds he's gunning for trouble when he swaps the mean, IED-strewn dusty roads of Afghanistan for the mean streets of, erm, Southwark.
Still wired after being ambushed by Taliban guerillas, he struggles to keep a lid on provocation from feral local gangstas and deals with post-traumatic stress by punching the walls of his flat.
Unable to land a job, he's tempted by the opportunity for undercover surveillance work offered by shady government spooks Brian Cox and Tony Curran.
He gets the gig of shadowing a suspected cell of Al-Qaeda terrorists and is subsequently requested to make contact with Adi Bielski's panicking infiltrator and ship her out.
Director Matthew Hope's strongest suit is Kebbell, who first impressed in Dead Man's Shoes opposite Paddy Considine and imbues Miller with a jittery menace reined in by a brooding sense of justice.