Pipped at the post by Ken Loach's The Wind That Shakes the Barley at Cannes but now the recipient of the Golden Globe for best film, Alejandro Gonzalez Iñarritu's extraordinary movie, written, like Amores Perros and 21 Grams, by Guillermo Arriaga, is his most ambitious yet.

It takes in the whole world as it illustrates, in four separate but linked segments, how small mistakes often have larger consequences and how poorly an ever-shrinking universe manages to communicate. This is a powerful and brilliantly made film. There are, however, moments when its pretensions get the better of it.

The initial sequence has two children in a mountain village in Morocco, given a gun by their father to protect their sheep from predators, who fire a round at a tourist bus. On it are an American couple Richard and Susan (Brad Pitt and Cate Blanchett) on holiday to repair a shaky marriage. She is badly hurt by the stray bullet and her fight for treatment leads to an international incident with terrorist overtones.

When the couple can't get home, their two children, in the care of a kindly but illegal immigrant (Adriana Barraza), travel into Mexico with her nephew (Gael García Bernal) to attend a wedding. A border patrol chases and catches them on their return. Finally, in Japan, a deaf-mute teenager (Rinko Kikuchi) indulges in risky sexual escapades after the suicide of her mother. It turns out that her distant father sold the gun to the Moroccan family in the first place.

There are scenes everywhere in the film that illustrate how good a filmmaker Iñarritu is. For instance, the desperate-moment when Susan is hit by the bullet and Richard realises that the bus wants to depart and there is only a vet in the village to help her, apart from an old woman who offers opium.

The performances here are as intense as the film-making, as is the portrait from the less experienced Kikuchi as Chieko in the final episode.

But if the film seems intent on illustrating the butterfly effect, which has it that the flapping of the wings of a butterfly can cause a tornado half a world away, it doesn't entirely convince. It's much better on the lack of understanding we have for each other and the chaos that can result.

The film, made in English, French, Spanish, Japanese, Berber and Arabic, also makes its point about the difficulties of language most effectively. But while it never loses its grip, its ambitions come across as hollower and more grandiose than its director and writer intended. There's not much optimism about human existence to be found in Babel, even though most of us in real life, I believe, actually have as much good luck as bad.