Line Of Duty's Jed Mercurio says he keeps things flexible when writing the hit show - and storylines can change depending on the stars' schedules It was revealed during Sunday night's eagerly-anticipated finale that The Fourth Man - also known as H - was Superintendent Ian Buckells Initially portrayed as a hapless character, it was revealed that he was in fact the criminal mastermind who had formed the secret network of bent coppers Many clues appear to have come to light since the show hit screens in 2012, including a real life note from actor Nigel Boyle, who tipped off fans in 2017