In the early 1980s in Belfast, Sean Duffy, a conflicted Catholic cop in the Protestant RUC (Royal Ulster Constabulary), is recruited by MI5 to hunt down Cormac McCann, an IRA master bomber who has made a daring escape from the notorious Maze Prison. In the course of his investigations Sean discovers a woman who may hold the key to Cormac's whereabouts; she herself wants justice for her daughter who died in mysterious circumstances in a pub locked from the inside. Sean knows that if he can crack the "locked room mystery," the bigger mystery of Cormac's whereabouts might be revealed to him as a reward.