It's the early 1980s in Belfast. Sean Duffy, a conflicted Catholic cop in the Protestant Royal Ulster Constabulary, is recruited by MI5 to hunt down Dermot McCann, an IRA bomber who has escaped from the notorious Maze prison. Sean discovers a woman who wants justice for her daughter who died in a pub locked from the inside. Sean knows that if he can crack the 'locked-room mystery,' the bigger mystery of Dermot's whereabouts might be revealed.