Neptune and Shark are hired by MI5 to follow an IRA member-Shamus Murphy, in the hopes he will lead them to the head of their organization, Sean O'Leary. Initially they were reluctant to accept, their memories of the case they undertook for MI6 still fresh in their minds. However, once they learn that the IRA is planning a strike against the police, they know they can't refuse. They have too many friends amongst the police to want to see any of them...