It's 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 Dermot 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 Dermot'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 Dermot's whereabouts might be revealed to him as a reward. Meanwhile the clock is ticking down to the Conservative Party conference in Brighton in 1984, where Mrs. Thatcher is due to give a keynote speech.