The human need to connect is one of the fundamental values that we share. The desire to bond, to find in others an estimation of our own value – whether they be family, friend or lover – is something that drives all but the most psychopathic in their daily lives. For Tom Spicer, though, the person with whom he most wants to connect is Metallica's Lars Ulrich. "Wanna meet Lars," he's said to his sister, Kate, and brother, Will, for the best part of two decades. "Wanna meet Lars." It's an unlikely dream for anyone to fulfil, but it's an even greater challenge for Tom, who suffers from Fragile X syndrome – "a sort of autism with bells on", according to his sister – that has led to him living in a care home, spending his days making animal bedding from old paper. 

Mission To Lars is a documentary that tracks the journey undertaken by Kate, a lifestyle journalist, and her filmmaker brother Will (who co-directs, with James Moore) in their attempts to grant Tom's wish. He is, they reason, "entitled to one big adventure", and, after years of drifting apart from him as they follow their own careers, they decide to undertake an ambitious and, some might say, foolish plan to remove Tom from his Devon institution and fly him to the US where Metallica are touring. Their intentions are honourable: after years of Tom's mantra they feel certain that this could prove to be the trip of a lifetime for them all, something that will bring them closer together once again and simultaneously offer Tom an unforgettable experience. But, though they perhaps don't realise it, the trip also offers an opportunity to ease the consciences of the two negligent siblings, and it's the tension between these two conflicting demands – to please Tom, and to make themselves feel better – that ensures this simple film is so powerful, humorous and moving.

Mission To Lars centres upon the stuff that Hollywood is founded on, but this isn't Hollywood, and before the pair can even leave British shores, Tom has gone missing. His disability, it soon becomes clear, makes him a stubborn individual, and neither sibling fully understands how to handle his behaviour. Having somehow succeeded in getting him across the Atlantic, they then face lengthy drives in a vast RV as they follow Metallica from Las Vegas to Sacramento and finally Anaheim, and their battle to win Tom's confidence gives rise to strain within their own relationship. Whether or not they can get close to Lars Ulrich becomes almost secondary to whether or not they three of them will actually survive the trip together as a functioning family.

What makes Mission To Lars fascinating is that it allows us to see how complicated the desire to connect really is. As Tom struggles to comprehend the possibility that he may actually meet his hero, Kate and Will become increasingly frustrated, both with him and with one another. Their potential failure to turn this into the fairy tale journey that they wanted becomes a source of increasing anxiety: making Tom happy will actually make them happy, and this grand gesture, it seems, is the only way they can show how much they care about him, but unless he cooperates, they can't feel good about themselves in the manner they had hoped.

Their experience throws into sharp relief the reality that life is rarely as simple as we wish. You can't force someone to accept your kindness if they don't want it, just as you can't provoke someone to fall in love with you simply by falling in love with them. Whether Tom is disabled or not is of little importance (and when it is, it's treated sensitively, with humour and affection). The problem is that there's a danger that all Kate and Will have really done is attempt to 'buy' Tom's trust and affection, and the fear that it's all been in vain shows in the slow deterioration of Kate's nail varnish, which she nervously scrapes off as the trip continues. Of course Tom has spent years begging to meet his hero, so it's hardly surprising that they expected him to be more excited about the prospect. But, then again, the possibility exists that this ambition was never meant to be fulfilled – it simply gave Tom something to which to aspire. Maybe, in their guilty and perhaps desperate need to bond with their brother, Kate and Will fixated on it more than Tom ever did.

As this curious road trip continues, Kate and Will realise that uniting Tom and Lars is perhaps less important than simply trying to understand their brother. His regular refusals to partake in plans that they have made especially for him suggest that he feels they've failed to realise what he actually needs – contact and appreciation – and his frequent repudiations test them to the limit. Frantic calls to their family reveal that there are far more important things they need to appreciate about him and his condition, and when they stop off to meet a Fragile X specialist along the way (whose car number plate is rather amusingly revealed as FRAGL X) it appears to be the first time since they gathered that Tom really feels like someone knows how he feels. The dream that mattered to Kate and Will, it starts to become clear, was less that Tom meet Lars, and more that they could say they made it happen.

As the film nears its climax, it becomes obvious that any failure to track down Lars will mean a great deal more disappointment to Kate and Will than Tom. They realise that he's anyway perhaps unable to handle such a momentous meeting, and stress is written all over Kate's face as she makes frantic phone calls to the band's management. Tom, meanwhile, continues blithely to wander around campsites wearing headphones or to sit quietly in the back of their bus staring out at the scenery. None of this really matters to him, it seems. He could be almost anywhere.

There's no need to spoil the ending, of course. In many ways it's as irrelevant to the film as Lars' whereabouts are to Tom. What matters is simply that, for a short period of time, the three of them are brought together by a common goal, that of sharing an experience – any experience – without it being burdened with meaning. True friendship and love don't need to be forced: the value we hope that others will find in us is only meaningful if it is unprovoked. Tom somehow recognises that meeting Lars is actually to all intents worthless. All that matters was that he had a dream. It takes this chaotic and deeply touching journey to show Kate and Will that, in trying to show their brother that they love him, they were also looking for his love.

"For as long as I can remember," Kate narrates at the start, "I've had recurring dreams about Tom. In them, we have long conversations, make plans, do stuff. They're good dreams." That's all that was needed: conversations, plans and stuff. That and the knowledge that – whether or not such plans are realistic – there are people around us who care enough, and value us enough, to want our dreams to come true.

Mission To Lars premieres on Wednesday June 6 at Hackney Picturehouse, followed by a q&a with the directors. It will then be shown at Picturehouse Cinemas nationwide. More information can be found at the film's website.