It's 1989 and fresh-faced graduate Angus Thoroughgood has just become Z325 Constable Thoroughgood and has exchanged the cosy cloisters of Glasgow University for the broken streets of Lennox Hill, 'Z' Division, the frontier posting that no rookie wants. He soon discovers his shift is full of veterans of the Northern Ireland troubles who are intent on ruling 'The Hill' by a brutal set of rules they have transferred from their time in the Province. Recruited by a new Internal Investigations Unit, he discovers that Glasgow's criminal overlord Bobby 'Mojito' Dawson is a member of a deep cover special Army Unit and that his foes in the escalating gangland feud known as The Ice Cream War, the McGuigans, have strong links to the IRA. When Mojito recruits a deadly assassin from across the Irish Sea to bring the Ice Cream War to an end, Thoroughgood finds himself in a race against time before Glasgow is blown to smithereens.