Those who recall the illuminating despatches of the journalist Robert Fisk, the official persecution of the British intelligence officer Colin Wallace and other disclosures of skullduggery by the security forces in Northern Ireland in the 1970s will not be totally surprised by the contents of this book. But others, either born since then, or having allowed their memories of The Troubles to be blurred by indifference, boredom or distaste, will be profoundly shocked by The Ultras.

Employing an increasingly common technique, the author focuses on the life, career and assassination of a British Army officer, Captain Robert Nairac, and, with the help of hindsight, good research and intelligent surmise, creates a novel of unmitigated menace. Nairac, unlike his posthumous, media-cultivated image as a gallant officer, was a psychopathic braggart who conspired with other psychopaths from the loyalist “shadowlands” in a kind of brotherhood called the Ultras to bump off both innocent Catholics (for example, members of the Miami Showband) and culpable ones (such as John Francis Green, a prominent IRA man who lived on the southern side of the Irish border).

McNamee renames most of the actual cast (many of them now dead). Yet by flicking through reliable factual histories, such as Tim Pat Coogan’s The Troubles: Ireland’s Ordeal 1966-1995 and the Search for Peace, one can quite easily identify them. What one cannot do, however, is conclude with certainty how or where Nairac’s killers disposed of his corpse, which has never been recovered. According to popular belief, reflected in McNamee’s narrative, the unfortunate captain ended up either in a peat bog or in a meat factory’s grinder. For that reason alone, perhaps, the Nairac story is one of the most abiding and tantalising mysteries to survive the Ulster conflict.

Any attempt to penetrate it entails an encounter with what McNamee describes as “clandestine governance, the dark polity”: collusion between MI5 and the hard men who drink to excess and oil their guns fastidiously, rogue RUC men and others of vaguely fixed affiliations and vaguely directionless intent. In this fictionalised account the reader shares a protagonist’s consciousness “of conspiracy getting out of control ... spiralling downwards”, as MI5 agents strive to undermine peace efforts by MI6.

These events occurred during the most vicious blood-letting by the IRA, and the covert war against the terrorists had become increasingly complex and uncertain. Dirty tricks were the order of the day. These fed into the rumour machine which, in turn, provoked an almost daily dèmenti from the authorities. Nobody believed the denials. Many denied their beliefs. Out from this murk emerges Blair Agnew, a blemished former sergeant of the Royal Ulster Constabulary who has associated with the Ultras. A quarter century after Captain Nairac’s disappearance, Agnew attempts to face up to his own past role in the conflict and to make sense of the Nairac affair, filling his caravan with thousands of newspaper clippings, government statements, secret documents, pictures of dead people, while trying to cope with his anorexic teenage daughter. In McNamee’s electrifying story, the murk thins, thickens, thins again, but never clears.