You have to turn to the brief interview with director Anne Fontaine that is the sole extra on this DVD release to discover the real source of her film Reinventing Marvin. Though Fontaine and Pierre Trividic’s screenplay is credited as original, it draws heavily – Fontaine calls it a “free interpretation” – on Edouard Louis’s bestselling 2014 autobiographical novel The End of Eddy, which told the story of his growing up in the French provinces in an environment profoundly hostile to his emerging gay identity.

It’s an undeniably powerful picture of a youthful outsider, one for whom lack of understanding at home, in an unsympathetic working-class family, proved almost as cruel as the school bullying that his sexuality evoked. But Louis offered no clue as to how he came to escape a world that could so easily have trapped him, no suggestion of the process – and, crucially, who helped along the way – that saw him evolve into who he is today, with a confidence that was able to overcome such beginnings. In other words, just how the “reinvention” alluded to in the film’s title actually took place.Fontaine has filled in that gap by positing an imagined concept that the Louis figure, here named Marvin Bijou – the awkwardness of that surname, translated as “Jewels”, seems horribly ironic for a context that is anything but sparkling – found his path out of that desolate early milieu through theatre. Early encouragement from a sympathetic schoolteacher is fortuitously followed by engagement with an intuitive stage director-coach, who draws Marvin both out of himself and into the Parisian gay scene. Never entirely losing his shyness, his involvement in that culture grows, encouraged by a largely benevolent sugar daddy figure who moves in circles of which the youth could once barely have dreamed.

With the film’s action framed by the young man’s development of his life story into a stage script, this unlikely dream narrative is completed by acquaintance with Isabelle Huppert, no less, who plays herself (pictured above, Huppert with Finnegan Oldfield). What else remains but to put on a show with her that will bring him fame, as well as a chance to revisit, and in some way make peace with, those childhood roots? There’s barely a cliché of the well-trodden “self-realisation through art” formula left untouched, with the gradual childhood absorption of the young Marvin (an absolutely winning performance from Jules Porier, main picture) in the challenges of his chosen artistic path familiar from the likes of Billy Elliot.

Huppert plays beneficent with her customary aplomb

But Fontaine's story seems somehow to predicate the world of her grown-up protagonist; he later changes his name to Martin, adopting the surname of that first supportive teacher, too. Finnegan Oldfield is undeniably attractive in that role, but aesthetic engagement comes to dominate over emotional involvement as the story progresses. Fontaine talks of needing to find actors who could “fit with each other”, and there is indeed an almost uncanny echo between the shape of their faces, especially evident in a shyness around the mouth, of the two; she cast Oldfield first, but the essential presence here is surely Porier, who brings an absolute, open-eyed freshness – one that can be almost agonising to witness – to the world that he negotiates with such difficulty. Reinventing Marvin hits home when dealing with the pain and awkwardness of its early scenes, far more than the mannered direction, rather overstretched at 115 minutes, which it takes later en route to a rather portentous finale.

It may be a story about a kind of redemption, but those childhood scenes remain more memorable, albeit in an almost gruesome way, than anything that follows. Gregory Gadebois is tremendous as Marvin’s indolent, overweight father, who oversees his hapless prolo household with alcoholic brusquerie, a characterisation that leaves us to question the film’s suggested resolutions. Vincent Macaigne, attractively sympathetic as the youth's drama teacher Abel, is never more convincing than when he’s disabusing his moping protégé about the lasting interest of his incarnation as “tormented working-class fag”. Huppert plays beneficent with her customary aplomb, even as we wonder whether she would be moving in these circles in the first place.

• Read more film reviews on theartsdesk

Overleaf: watch the trailer for Reinventing Marvin