In the wake of events at Charlottesville, Virginia, Kathryn Bigelow’s historical drama Detroit could hardly be more topical. But then you could argue that US movies about racial bigotry will always have a contemporary resonance – because it’s a problem America seems pathologically unable to solve. Bigelow, a brilliant film-maker capable both of superior trash (Point Break) and visceral, edge-of-the-seat war stories (The Hurt Locker, Zero Dark Thirty), takes on this chronicle of mayhem and murder with all the sincerity and commitment you could ask for, but she’s stuck with material that might have worked better as documentary: as polemic it’s unwieldy, and as drama it’s merely exploitative.