No recent movie has blind-sided Hollywood like The Blind Side. After an unexpected box-office triumph in the US this winter, this sweet-natured family story went on to win two Oscar nominations and then a best actress award for Sandra Bullock.

It was shocking enough that Sandra Bullock should be the lead of a film that was both successful and watchable. In recent years, the star of 1990s hits like Speed and Miss Congeniality has become more respected for her philanthropy - in 2006 she quietly donated a million dollars to help victims of the Asian tsunami and then a second million for the victims of the Haiti earthquake - than for her movies. With the arguable exception of Crash in 2005, she seemed to have developed a perverse instinct for choosing awful projects.

Even more of a shock was the fact that The Blind Side, without a drop of blood or a single explosion, immediately overtook the teen-oriented vampire flick Twilight Saga Full Moon at the box office.

It seems that it was able to do so partly because it attracted an audience that has tended to shun Hollywood films for many years, namely church-goers. Amazed to discover a movie in which the good guys happen to be practising Christians, they have turned up to see this uplifting pro-family tale in droves.

Hollywood generally does not make or like films that positively depict people with religious faith - unless that faith is suitably exotic and preferably non-Western. Yet The Blind Side centres on a church-going Southern family who are neither hypocrites nor outright monsters and whose story is all the more remarkable because it is true.

The film is adapted from the bestseller of the same name by Michael Lewis, (best known in the UK for Liars Poker, his memoir about being a bonds salesman on Wall Street).

The Blind Side is about the evolution of American football but its key chapter, Freak of Nurture, tells the story of Michael Oher, a troubled young black man from the worst ghetto in Memphis who was adopted by the Tuohys, a wealthy white Republican family, and who has now become an American football star.

At the age of sixteen, "Big Mike" (Quinton Aaron) had a 20-inch neck, 50-inch waist, a 58-inch chest and weighed almost 24 stone. He was also homeless. One of 12 siblings, his mother was a crack addict and his father had been killed in a drug deal gone wrong. He was almost completely uneducated and his IQ tested at the low end of the scale. His first big break was admission to a private Christian high school but he was still essentially without a home or hope.

The person who came to his rescue was one Leigh Ann Tuohy (Bullock), a sassy, blonde interior designer who was driving her family home from the same school one winter night, saw a shivering Big Mike trudging through the rain in only a T-shirt and shorts, and invited him into her home.

The relationship that ensues, and which is at the heart of the film, is a kind of light-hearted mother-son romance, as Bullock's voluble, frighteningly confident Southern pocket- rocket bonds with the strange giant boy who will not talk about his past or much else, but who turns out to have greater academic and athletic potential than anyone realises.

It is probably Bullock's best performance ever, though her twangy Tennessee accent is not quite as good as Kathy Bates' who plays Miss Sue, the tutor who the Leahys engage for Michael, or as the one employed by country singer Tim McGraw who plays Leigh Ann's genial husband.

The strange thing about The Blind Side is that it manages to pack a remarkable emotional punch despite suffering from a kind of pat predictability and some awkward writing by screenwriter-director John Lee Hancock.

Though it generally avoids scenes of saccharine sentimentality, it stays very much on the surface of things. There could have been so much more about how Michael and his new family dealt with their differences, how he was treated at school, how Leigh Ann was rebelling against the racist mores of her own family, or how Michael turned out to have genuine academic ability.

Another problem is the film's failure to show why Michael Oher was such a catch for high school, college and professional football teams. In the film he is just an unusually tall, fat boy with a certain amount of brute strength, and if you did not know better you would imagine that sheer size is all it takes to be a football star. You certainly get little sense of the young man's extraordinary combination of speed and agility as well as strength.

(Another oddity is that alone among the characters depicted in the film, the real-life Michael Oher is much better looking than the actor who plays him.)

Yet The Blind Side is a remarkably moving and uplifting film. And in its unpretentious way it is full of surprises, whether it is the literary references that range from Tennyson to the children's book, Ferdinand the Bull, or the way that the Tuohy family think it is funny that they adopted a black son before they ever met their first Democrat.