Tony Marchant's film 'The Mark of Cain' tells three stories: that of the impossible task facing British soldiers in Iraq; that of the terrible cruelties they inflicted on Iraqi suspects; and that of a cover-up in the chain of command. The story is fictional but based on true events; however, there's less evidence in the real world for the cover-up than for the other two elements, and while it seems plausible, this aspect of the tale feels more "written" than the other two. And as always with Marchant, there's a careful and clever manipulation of the viewer's sympathies throughout: I never quite feel that Marchant gives me the space to form my own opinion. But it's strong stuff, well-performed and unflinching, even if it doesn't really say much in the end except that bad things happen in war.