Line of Duty episode 4 review ★★★★★

Jed Mercurio’s writing attracts countless theories week by week. It’s part of the fun of watching Line of Duty unfold, seeing tiny details and exploding their significance. Very often the theories are correct, with eagle-eyed viewers posting their predictions online.

What they can never predict is the tone Mercurio pulls them into. It’s difficult to know how a perilous action scene or a violent confrontation will take form, which is where Mercurio unleashes his vicious surprises.

Episode four brakes and steers into a different tone, not only raising the emotional stakes but shuddering into something close to a horror movie that feels all too real. This is series 5’s most brutal episode yet.

After the events concluding episode 3, Ted (Adrian Dunbar) is more aggressive than ever

Last week glanced into the true evils facilitated by the OCG, led by John Corbett (Stephen Graham), chiefly in their trade of sex slaves. AC-12 freed those slaves from an overlooked Print Shop.

McQueen (Rochenda Sandall) and her goons visit a man with downs syndrome who keeps an obedient eye on the Print Shop for the OCG. It’s dark in the flat, one goon is rather unpleasant. But that’s nothing compared to what they find in the freezer, so bluntly and blithely revealed. There’s no pussyfooting here: true horrors bleed into these criminals, their immorality knowing no bounds.

The tension has also never been higher, sharply fiddling with the characters’ greatest weaknesses. After the events concluding episode 3, Ted (Adrian Dunbar) is more aggressive than ever. Mercurio is careful not to create too much melodrama from Ted’s pain, with Dunbar’s emotionally tortured performance being more than enough. He’s in a genuinely fragile state we’ve rarely seen before and it’s hard to watch him break.

Ted's fling with Gill (Polly Walker) is another annoying misstep

That sadness and fear and loneliness drifts to Corbett, whose undercover guise is starting to slip. At times, it’s like Dunbar and Stephen Graham are competing in their exhibitions of upset, with Graham only just beating him. There’s another scene when Corbett phones his wife, and watching Graham’s face is like watching a tortured soul splitting in half.

It’s difficult to justify all of Corbett's courses of action, but it’s easy to understand them. It’s what makes him such a morally discordant character and one of Mercurio’s finest creations.

Ted’s fling with Gill (Polly Walker) is another annoying misstep, but it’s only a slight stumble in an otherwise impeccable episode. This week proves that Mercurio is never afraid to push his plots and characters to their darkest limits. Make sure you're prepared.

Line of Duty continues Sundays at 9pm on BBC One