King of the New Wave: BFI salutes the brilliant, groundbreaking French film-maker François Truffaut

If there is one scene that sums up the work of the French film-maker François Truffaut (the subject of a major retrospective at the BFI next month), it's a moment midway through his 1976 film, Small Change, about children growing up in a small town in France. A baby boy called Gregory is left alone in a high-rise apartment. He is playing with a pet kitten that refuses to come in from the window ledge and then gets stuck. Gregory playfully tries to rescue the kitten, loses his grip and falls downward to his certain death... but he doesn't die. "Gregory went boom!" the little youngster tells the adult onlookers as he dusts himself off on the ground dozens of floors below. His mother faints. Gregory makes no fuss. Nor does Truffaut. In his universe, no harm should ever be allowed to come to children. The film-maker was, as one friend described him, "a kind soul" and "a treasure trove of tenderness".

It's worth bearing this description in mind when you consider Truffaut's first major intervention in film culture – his 1954 Cahiers Du Cinema article, "A Certain Tendency of French Cinema". This was a ferocious polemic against what he dismissed as "Le Cinema Du Papa". The young critic tore into what he called "the tradition of quality", excoriating film-makers who specialised in tasteful literary adaptations that won prizes at festivals and were lapped up by foreign critics. The reaction to his broadsides was similar to that of the Hollywood establishment toward Ricky Gervais when the British comedian took potshots at them on Golden Globe night. Leading French screenwriters and film-makers derided and dismissed the young upstart and confidently predicted that he would sink back into oblivion. Of course, the reverse happened.

There was more to Truffaut's revolt than dismay at what he considered to be the hidebound style of movie-making epitomized by Jean Aurenche and Pierre Bost, the most famous screenwriting team of the era, and by directors like Claude Autant-Lara and Yves Allégret. "A great many of the producers of the time had collaborated with the Germans during the war and so there were all sorts of reasons to try to eliminate their stranglehold and, incidentally, the stranglehold of the unions on French film-making," the film-maker Marcel Ophüls later observed of Truffaut's essay. Another element was generational: Truffaut was part of a younger generation of cinephiles – the so-called "Hitchcocko-Hawksians" – who much preferred Hollywood B movies to self-conscious French literary adaptations.

It was later claimed that Truffaut had seen 4000 films between 1940 and 1955. It was not hard to see why he had turned toward cinema with such passion. The film-maker (born in 1932) had had a famously unhappy childhood. Movies were his way of escape. Antoine de Baecque and Serge Toubiana's exhaustive biography of Truffaut reveals how closely the director identified with Charles Dickens and with Dickens's David Copperfield in particular. Like Copperfield, he never knew his biological father. Truffaut may not have worked in a blacking factory (as the youthful Dickens once did), but he felt a similar sense of injustice about his upbringing. His mother, Janine, didn't pay attention to him. The grandmother to whom he was so close died when he was only 10. His stepfather, Roland Truffaut, was more interested in mountaineering than his new son.

The future auteur was the quintessential latch-key child. At weekends, while his parents went off mountain climbing, he was left on his own with pre-cooked meals. As his stepfather quickly surmised, the young Truffaut preferred "scooters and the movies" to the great outdoors. Three novels a week and three movies was the cultural diet he adhered to with the utmost strictness throughout his adolescence. Balzac and Dumas were particular favourites. He and his friend Robert Lachenay started their own film society when they were still teenagers and used fraud to try to make it profitable. They claimed that Jean Cocteau was coming to one of their screenings. A big audience turned up and paid to see him, but he never showed.

De Baecque and Toubiana offer an awful description of Roland Truffaut shopping his wayward stepson to the police. The young Truffaut was a petty thief who had accumulated huge debts. Roland tried to pay these off, even as he abandoned him to imprisonment. In a misguided attempt to go straight, Truffaut enlisted in the army, but then went Awol rather than risk being posted to Indochina. This resulted in imprisonment. By his early 20s, he had contracted syphilis several times and had befriended the underground writer Jean Genet, whose The Thief's Journal he regarded as a manifesto for his own life.

Truffaut made his reputation as a film-maker with The 400 Blows (1959), a film about a famously unhappy adolescent, Antoine Doinel (played by Jean-Pierre Léaud.) By then, he had already developed "la politique des auteurs", a highly influential approach to film criticism but one as much rooted in his own whims as in any rigorous theoretical basis. He used auteur theory to defend film-makers he admired (from Abel Gance to Hitchcock) and to bludgeon those he did not. Truffaut's apprenticeship as a film-maker was wrapped up with his career as a journalist and critic. He befriended directors he admired (notably Jean Renoir and Roberto Rossellini); interviewed them at length; followed them around and tried to work with them. As far as financing his first feature, it helped that he had married Madeleine Morgenstern, daughter of one of the most powerful distributors in Paris.

It's instructive to compare the Doinel of The 400 Blows with the character who reappeared (again played by Leaud) in later films like Stolen Kisses and Love on the Run. The teenage character first seen on screen was a little warrior: hostile, fierce and introspective. As a young adult, Truffaut's cinematic alter ego was very different: a whimsical and romantic man about town.

It was clear that Truffaut identified strongly with Lucien De Rubempré, the ambitious protagonist of Balzac's great novel Lost Illusions, who had affairs and made his name as a journalist in Paris but betrayed those closest to him and sacrificed his ideals in the process. It was also apparent – and very ironic given his attacks on literary adaptations in "A Certain Tendency..." – that he loved literature as much as he did movies. In the late 1960s, when his friend and colleague Jean-Luc Godard embraced Maoism and began to make self-reflexive, Brechtian movies, Truffaut continued to turn out well-crafted features with often surprisingly conventional narratives. Jules et Jim (1962), arguably his most celebrated movie, was based on a novel by Henri-Pierre Roché, a by then elderly writer and artist he had befriended.

A new documentary, Two in the Wave, directed by Emmanuel Laurent and scripted by De Baecque, tells the story of the friendship and rivalry between Truffaut and Godard – the key figures in the French nouvelle vague. They first met in the late 1940s and – at least until the rupture of 1968 – their careers progressed in tandem. "When Truffaut made Jules and Jim, Godard replied with A Woman Is a Woman, also a ménage à trois story," Laurent recalls. "Or, when Truffaut made The Soft Skin, Godard did A Married Woman. It was always the same topic, the same subject. They kept talking to one another through their films, relating to one another."

The two directors even used the same actor, Jean-Pierre Léaud. "Godard wanted to have a substitute of Truffaut in his films. We may say that Truffaut gave birth to this actor as a boy, but Godard turned him into an adult. He (Léaud) rose to political consciousness."

Another trait the film-makers shared was their womanising. The comical beginning of Truffaut's 1977 feature The Man Who Loved Women, when female mourner after female mourner turns up at the funeral of the main character, was the director's in-joke about his own romantic adventures and misadventures. The list of women linked with him was almost as long and included such figures as Catherine Deneuve, Leslie Caron and Fanny Ardant.

The friendship between Truffaut and Godard imploded in the wake of Day for Night (1973), Truffaut's Oscar-winning film about an obsessive filmmaker. Godard thought Truffaut a sell-out. Truffaut thought Godard an impostor and a "piece of shit on a pedestal". There was to be no rapprochement before Truffaut's untimely death, aged only 52, in 1984.

Truffaut was a wildly contradictory character. Given that so much of his work was rooted in his own life, it was little surprise that his movies were contradictory too. As a critic in the 50s, he was the "young thug" of film journalism – or, at least, that was how he was viewed by the older directors he assailed. He was at the head of the New Wave. He was ambitious to the point of ruthlessness. At the same time, a tenderness and yearning ran through his work, too. The unhappy child and delinquent teenager never lost that sense of compassion or that instinct always to root for the outsider.

The Truffaut season runs at BFI Southbank from 1 February to 12 March. 'Two in the Wave' is released on 11 February