Fri, 30 Apr, 2021 - 11:20 Who is H? We're about to find out as season finale of Line of Duty airs this weekend Mystery deepens as a number of characters remain under suspicion in the hunt for 'the fourth man' Line of Duty stars Vicky McClure as DS Kate Fleming, Martin Compston as DS Steve Arnott, and Adrian Dunbar as Ted Hastings are believed to be among the few cast members who know the identity of H. Kelly Macdonald has described Line of Duty's famously secretive creator Jed Mercurio as "troublesome", and revealed only the "top cast" know the identity of H ahead of the series finale.