Woodstock 1969 lives on as the epitome of cool. Images of free love, Jimi Hendrix's guitar and Joan Baez, The Who, Janis Joplin et al were etched into posterity by Michael Wadleigh's classic 1970 documentary. Forty years down the line, Brokeback Mountain director Ang Lee has decided to celebrate the geeks that made the concert possible. It's the most fun film in competition at Cannes so far.

It's hard to imagine a more unlikely hero than Elliot Tiber, who wrote the book Taking Woodstock: A True Story of a Riot, a Concert, and a Life, which has been amusingly adapted by the screenwriter James Schamus.

Demetri Martin, replete with a hooked nose and gaunt features, plays Elliot, one of life's unhappy nice guys. He could have walked off the set of American Graffiti. He desperately needs to escape his adolescence and break ties with his overbearing parents, especially his mother (Imelda Staunton). They run a failing motel in White Plains, Woodstock. Ang Lee embraces the clichés, so it's violet dresses for the girls, and long hair and amusing facial hair for the boys – when they're actually wearing clothes, that is.

As Elliot tries to come up with ideas to save the motel and re-invigorate the community, we meet an array of kooky characters: his proud father Jake (Henry Goodman), the mad experimental theatre group leader Devon (Dan Folger), the chocolate milk-making neighbour Max (Eugene Levy) and the best friend, Billy (Emile Hirsch), a Vietnam veteran who looks like he's just out of his nappy. Lee shows that there were freaks in this town way before the hippies arrived. Elliot's desperation sees him call the producer Michael Lang (Jonathan Groff) when he hears the neighbouring town of Walkill won't licence the concert. And the rest, as they say ...

We're soon into a culture clash comedy as the townsfolk adapt to more than a million hippies, only outdone by the arrival of Liev Schreiber as a cross-dressing ex-marine. Alas, the fun does not last. Once the concert starts and Elliot has his inevitable LSD trip and introduction to free love, the film drops the comedy for a needless coming-of-age denouement in which Elliot breaks from his parents. It would have been better had the movie ended when the concert began.