Kate and Will Spicer's brother, Tom, has an inherited learning disability, known as Fragile X Syndrome. He is also a massive fan of Lars Ulrich from Metallica. Tom's siblings made a promise that they would get him to meet Lars.

Dream journey backed by basic beats.

BRITISH FILM FESTIVAL: This is the kind of film that is hard to knock because, as therapists and the newly enlightened like to say, it is coming from a good place. Any criticism of technique and style is likely to be condemned as harsh, and perhaps beside the point; the filmmakers have even pledged any profit to support Mencap, a British mental health charity. It’s a documentary, a story of family, and familial love. It wants to be positive about something – autism – that a lot of folks don’t understand or would rather ignore because they find its character and consequences too threatening and uncomfortable to even contemplate. It’s friendly, warm and decent. Still, as a piece of filmmaking, Mission to Lars sits at about the level of a poorly made reality TV show. And that makes puny its sunny idealism.

It’s like they resorted to convention as a way to ease their own discomfort

Yet there are choices here that have real promise. The tone begins as intimate, almost confessional. The film’s producer and narrator is Kate Spicer, a news print journalist. In a voice bleeding with regret, she explains how for years she has, in her way, neglected her middle-aged autistic brother Tom. She has another brother, Will, a filmmaker. He doesn’t speak for himself, but Kate talks for him; he’s been a poor brother to Tom too. The movie and its making is an attempt to mend fences, repair damage before it’s too late for these adult children. Alas that thoughtful tone is abruptly abandoned as the film’s plot gimmick gets under way.

Kate and Will propose a plan. For as long as Kate can remember, Tom has been a devoted fan of Metallica. 'I wanna meet Lars," has been Tom’s mantra. Tom’s dream is to shake the hand of the famed metal band’s magnificent and diminutive drummer, Lars Ulrich.

The story of the film is how Kate and Will try to make Tom’s dream come true. Kate, who takes on the film’s role as on-screen 'anchor’, organises a road trip to the US, where Metallica are on tour. She discovers that there are three chances for the family to meet with the band. What little suspense the film has doesn’t come out of her efforts to negotiate a meet between Lars and Tom – and that’s because Metallica’s management seem, well, really nice – but rather the family set-up, which is fraught with tension.

This is because, in the mild therapy-speak of the film, the trip has 'its challenges’. Tom, a bespectacled guy with a kind smile, who looks much younger than his 40 years, suffers from what is called Fragile X syndrome, an intellectual disability that Kate helpfully describes as, 'autism with bells on". I’m not quite sure what that means, but when I looked it up to get a clinical description, I found its behavioural characteristics included 'hand-flapping’ and 'social anxiety’. It also emerges in obsessive behaviour.

The film begins in England where we meet the Spicer’s mum and dad and carers – who caution the filmmakers on Tom’s panic-attack 'triggers’ – and we catch a glimpse of his life in a folksy communal care home. But most of the film’s action takes place on the road in the USA where we travel with the Spicer’s and crew to LA and Las Vegas. The film team hire a mobile home and, at least for Kate and Will, it becomes a guilt-trip, where long suppressed resentments emerge. There are lots of shots of Kate and Will trapped in the van bickering while Tom sits in quiet, an unhappy guest in his own private world. (The trip, of course, disrupts the lifestyle that helps to keep Tom anxious-free and stable, like a solid routine and familiar people and surroundings.)

This stuff seems inherently dramatic. The hand-held camera and verite style promises something rich and deep but I couldn’t help but think the filmmakers have held back; the edit feels evasive, lacking in candour. Murky feelings of blame and unease with the situation and each other ripple on the surface and go unresolved; a dark history is hinted at and never unexplored. The sense that the filmmakers haven’t quite come to terms with themselves, the story and each other isn’t helped in the way that Will and co-director James Moore diminish much of the interest with a bland technique borrowed wholesale from TV travel shows. It’s like they resorted to convention as a way to ease their own discomfort.

But then this film has its fans. I suspect that’s down to its climax, which is a ripper. I won’t go into details, suffice to say, that Lars Ulrich seems a nice fellow and Tom’s joy in the face of his dream is like being caught in an embrace you don’t won’t to break with. Part of the reason why this bit of the film is so powerful is that the filmmakers let the images breath and speak as they unfold. We’re caught in the moment. I just wish the rest of the picture were as good.