At one point in the film The Blind Side (dir John Lee Hancock, 2009) Leigh Anne Tuohy (Sandra Bullock), a hard-nosed, well-coiffed and right-brainy Southern mother-of-two is having lunch with her regular girlfriends at their customary table in their usual white-china-and-clean-tablecloth restaurant in some privileged corner of Memphis, Tennessee. "You've done so much for him, he's so lucky," says one highly-primped, middle-aged matron from the far side of the flawless edge of her comfortable teacup. She's talking about how Leigh Anne has taken into her house a black youth who attends the same school as her son. Tuohy stares at her friend, offended as a proud mother by the implication of selfishness and as a Christian by the implication of self-interested charity.

"No," she intones frostily. "I'm so lucky because of what Michael has done for us." She means it and because she means it the audience is made witness to not only a perceptive mind but also a powerful but delicate realisation of life in the form of film. Just days after Leigh Anne, her husband Sean (Tim McGraw) and son Sean Junior (Jae Head) pick up a shivering Michael Oher (Quinton Aaron) as he walks on the cold, tree-lined street and bring him to their home, we get to see just what someone from the wrong side of the tracks can add to the bounty enjoyed by a well-off family like the Tuohys'. It's Thanksgiving and they are settling down to enjoy a festive repast while watching the football on TV. Michael takes a plateful but, instead of heading for the sofas in the family room to join the others, he retires to the more-formal dining room. Leigh Anne espies his lonely figure slumped in its massive solitude over the meal - and rouses the rest of her brood. They come to table where the food is carefully laid out. Then Grace is said over their conjoined hands. They give thanks together in a way that a poor boy from a broken family - Michael's mother is a drug-addict who has made children with possibly a dozen different men over the years and now lives alone - is best-qualified to appreciate.

Michael arrives at Wingate Christian School unexpectedly because his father wants him to get a good education. The father makes a point of first approaching the coach, who then petitions the board. But Michael has terrible learning problems and no home to go to. He has been sleeping on the couch of a friend but they kick him out. He survives by spending his evenings at a laundromat and relying on his rep as a gentle giant among his fellow students, including SJ, who brings Michael's plight to the attention of his mother as they drive home one rainy night in their top-of-range, late-model beige BMW. Initially, they just stop at the intersection and ask him where he's going. "I'm going to the gym," he replies. The car drives off a way down the street but Leigh Anne has second thoughts. "Turn around," she orders Sean. Pulling up beside the T-shirt-clad, lumbering youth, Leigh winds down the window. "The gym is closed," she tells him. "Why are you going there?" "It's warm," he replies. "Do you have a place to sleep tonight?" she asks.

Leigh Anne cares about Michael and takes care of him with the same protective ferocity she lavishes on 10-year-old SJ and teenage daughter Collins (Lily Collins). She learns what makes him tick. She talks to the teachers, one of whom, Mrs Smith the English teacher (Catherine Dyer) tries hard to understand why a youth of Michael's age has such trouble coping in the classroom, and who confronts the uncharitable complaints of the other teachers as they sit around in the Common Room after class. Like Leigh Anne, Mrs Smith appreciates Michael's intelligence and takes the simple precaution against failure of asking him test questions verbally, rather than just making him take tests like the other children, who are predominantly white.

The two women are joined in their endeavour later, when it becomes obvious that Michael has extraordinary sporting abilities, by Miss Sue (Kathy Bates), the tutor Leigh Anne takes on in order to ensure that Michael's grade-point average is sufficient so that he is not excluded from gaining a college sports scholarship due to poor academic results.

By focusing their attentions on Michael the three women ensure that he makes the grade. By tolerating his wife's peculiar obsession with this child of misfortune, Sean - a successful businessman who operates a string of fast-food restaurants in the city - ensures that the whole enterprise succeeds. They are undaunted by the bigotry of redneck parents shouting idiocies from the bleachers, the callous violence of the homeboys from Michaels' precinct, the incapacity of vision of coach Burt Cotton (Ray McKinnon), and the lack of compassion demonstrated by most of the school staff.

There's plenty of scope for over-egging the cake in a film with such a story, but the screenwriters - the director, John Lee Hancock, and the author of the book the film is based on, Michael Lewis - take pains to tone down the messaging to a low rumble of righteous indignation. It could have been shrill. It's not. It's more than a competent screenplay, it's a genuine and morally well-centred piece of craft that eschews easy pathos and plumps for a more robust set of ethical cognates. Lewis' book, The Blind Side, actually has two strands, one of which is the story of Michael Oher. Wisely, the writers focus on only the one strand and they make a masterpiece out of the raw material. Of course, Lewis is a consumate writer whose reputation preceeds him.