When a professor (Hopkins) with a painful secret stumbles over a politically incorrect phrase, and then takes up with a “trailer-trash” temptress (Kidman), he crashes headlong into scandal and disgrace. But where other recent melodramas (Mystic River, 21 Grams) have been humorless, morose, and contrived, Stain is good-humored, warm and glimmering with hope. Roth is honest about the consequences of wrongdoing, yet he coaxes us toward compassion rather than judgment. As the fallen professor, Hopkins gives a performance both gentle and fierce — he gets his career’s first love scene and, in a bold gamble, throws himself into a startling, joyous dance with a most unlikely partner. Benton’s refusal to distill Roth’s prose into platitudes and sap leads to a remarkably resonant conclusion for one of this year’s finest films.