This is book number 4 in the Sean Duffy series.
Belfast, 1985. Amid the Troubles, Detective Sean Duffy, a Catholic cop in the Protestant Royal Ulster Constabulary, struggles with burnout as he investigates a brutal double murder and suicide. Did Michael Kelly really shoot his parents at point-blank and then jump off a nearby cliff? A suicide note points to this conclusion, but Duffy suspects even more sinister circumstances. He soon discovers that Kelly was present at a decadent Oxford party where a cabinet minister's daughter died of a heroin overdose, which may or may not have something to do with Kelly's subsequent death.
New evidence leads elsewhere: gun runners, arms dealers, the British government, and a rogue American agent with a fake identity. Duffy thinks he's getting somewhere when agents from MI5 show up at his doorstep and try to recruit him, thus taking him off the investigation.
Duffy is in it up to his neck, doggedly pursuing a case that may finally prove his undoing.