During the Troubles in Northern Ireland, IRA members Brendan O'Hara and his son, Seamus, are ruthless executioners of informers. Brendan, betrayed by an unknown informer, is arrested but Seamus escapes and tries to put the past behind him. On his deathbed, Brendan forces Seamus to take an oath to kill the informer. But Seamus discovers he has a personal connection to the man who betrayed his father. Can he avenge his father without destroying himself?

In West Belfast in the late 1960s, Brendan O'Hara and his teenage son, Seamus, are drawn to the nationalist cause and join the IRA. Their obsession divides them from other members of the family, especially Seamus' mother, Ruth, and his gentle younger brother, Patrick. Ruth copes with her isolation through a close friendship with Linda, Brendan's sister-in-law, and a passionate and over-protective relationship with Patrick.

The family leave Belfast so that Brendan and Seamus can take part in the mainland campaign, where Brendan and Seamus become ruthless executioners of informers. Betrayed by an unknown informer, Brendan is arrested and sentenced to twenty years.

Seamus escapes the attention of the authorities and tries to put his past behind him. But he is horrified when, on his deathbed, Brendan makes him swear an oath to kill the informer who put Brendan behind bars. But Seamus discovers he has a personal connection to the informer. Tormented by guilt but bound by his oath, can Seamus avenge his father without destroying himself?