Darragh MacIntyre is the sort of investigative journalist television producers adore. He isn’t opposed to inserting himself into the action, and there will always be a scene or three in one of his reports in which he huffs about, chasing a lead like Hercule Poirot hunting down a murderer. All of his detective skills are required in The Secret Army (BBC One, Wednesday), a fascinating factual whodunnit featuring a lost IRA “documentary,” a potential CIA operative, and a cameraman who may or may not have worked for Mossad. It’s The Troubles meets a Tom Clancy novel – brimming with twists...