People across Britain are gearing up for the BBC's Line of Duty series finale tonight, with 12million viewers expected to tune in. The show sees the officers of police anti-corruption unit AC-12 pitted against the mysterious antagonist known only as 'H' or 'the fourth man', who heads up an organised crime syndicate controlling events from the shadows and potentially from within AC-12 itself. The series has gone from strength to strength since its release, growing from an initial audience of four million adults to more than 11million tuning in for the current series' penultimate episode. Earlier this week, the BBC released a teaser trailer promising an action-packed finale complete with a police standoff, the return of balaclava men and the discovery of a written note with THAT 'definately' misspelling.