Casting is everything. Gerard Kearns, who is best known for playing Ian in Shameless, has a face so sweetly expressive that he makes a writer's job easy - perhaps too easy. In an early scene from The Mark of Cain (12 April, 9pm), Tony Marchant's film about the exploits of a British regiment in Basra, a patrol came under fire. Mark Tate, 18 and on his first mission, hid behind a burnt-out car while the camera gazed on his froggy face, his mouth twisted in fear, his eyes too white and too wide. You ached for him. Later, when Tate (played by Kearns) was back in Britain and struggling with his conscience - he and his fellow soldiers had abused Iraqi prisoners, a crime for which he was now facing a court martial - we saw him in the bath, frantically trying to remove invisible Mesopotamian sand from his skin. This moment was painterly; with the arrangement of his limbs, you could hardly fail to think of the Crucifixion. But it was Kearns's face that stabbed at the heart: such misery branded on flesh so young.
Only later did I peel Kearns from the script and begin to see how very flimsy the writing was. The film was a game of two halves. The first was atmospheric, if rather familiar; the second morphed, in the cold light of day, into one-sided melodrama. Without Kearns, I doubt I'd have stuck with it. Marchant, having at first tried to equivocate, to balance bad acts with good, suddenly seemed unable to resist Making a Point, and that, predictably, was that the British army is in the contorted grip of institutional violence. Well, maybe so. But boy, did he labour it. It's one thing to show bullying and officers colluding to cover up acts of violence carried out on innocent civilians, quite another to portray squaddies beating up a colleague who has unhelpfully testified against them during court-martial proceedings. However squalid and secretive military justice is, this would not be allowed to happen; as Marchant himself showed in his script, the media are all over court-martials relating to events in Iraq. A pulped British face is no better PR for the army than a pulped Iraqi face.
I liked the way the film sprang to life at the beginning. Had this been Hollywood, we would have spent time getting to know Tate and his schoolfriend Shane (Matthew McNulty) - cue carefree shots of them with their families. Here, we were pitched straight into the chaos, with an immediacy that reminded you how alien a world it must be to mere boys, far from home for the first time. Filmed in Tunisia, these early scenes were just right: the white light, the choking dust, the women in their abayas seeming to float in the heat like strange bats. The dialogue, too, was acute. The soldiers passed some Iraqi boys who chanted "Manchester United" at them. "Typical United fans," said Shane. "They're 3,000 miles away." But some of the action felt borrowed - a bit of Saving Private Ryan here, some Platoon there - and there were plenty of clichés: the usual posh and not terribly bright officers, the snarling and tattooed regiment bully of whom all the other men were afraid.
This stuff filled me with a sense of foreboding and, sure enough, the balance began to tip. Soon everyone in a uniform, bar Mark and Shane, was corrupt and hateful. The men's corporal, who at first had seemed tough but decent - he made a moving speech telling his men how to carry empty coffins (empty because nothing was left of those who had died) - became a monster of bigotry and misguided loyalty. Mark and Shane, you understood, were going to be hung out to dry. Mark killed himself. Shane took his beating and was led away to a cell. Meanwhile, the man who had led the abuse of Iraqis, Corporal Gant (Shaun Dooley), was rewarded by his commanding officers for his silence and discretion, and promoted.
The Mark of Cain was screened a week later than planned, after Channel 4 withdrew it during the Iranian hostage crisis. Presumably, this was because the drama felt, at times, like anti-British propaganda of the kind that Mahmoud Ahmadinejad would probably quite enjoy. But the real issue, regardless of the timing, is whether Marchant's scathing portrait of British troops was a fair one. Even as a passionate opponent of the war in Iraq, I don't think it was simply unfair; it was unjust.
Pick of the week
This Is David Gest
22 April, 10.15pm, ITV1
Fly-on-the-wall series starring Liza's weirdly likeable ex-husband.
Cutting Edge: Killer Road
23 April, 9pm, Channel 4
A look at the A46 in Lincolnshire and those who have died on it.
The Diary of a Nobody
24 April, 9pm, BBC4
Start of Andrew Davies adaptation of the funniest book ever written.