"JUST think of it, " Guillermo Arriaga said three years ago.

"Somewhere in the world right now something might suddenly happen that without you knowing it may later change your life." This idea of reverberating consequences to random actions provides the trigger for Babel which, with Amores Perros and 21 Grams, completes a trilogy of movies Arriaga has written in collaboration with Mexican director Alejandro Gonzalez Inarritu. Each movie uses the same aesthetic vocabulary of fractured non-linear narrative, jumping backward and forward in time to tell very different stories. In Amores Perros and 21 Grams a dreadful car crash links otherwise unrelated characters without them immediately realising it. Babel widens the device beyond a single place - Mexico City in Amores Perros and Memphis, Tennessee in 21 Grams - to encompass four countries on three continents over 36 hours.

Babel starts in the Moroccan desert where a long-range rifle given by a grateful Japanese hunter to his guide falls into the hands of a mountain goat herder's two sons, who end up taking pot shots to see how far the bullet would go. One bullet hits a distant bus filled with western tourists, grievously wounding an American travelling with her husband.

The couple took the trip in the hope of patching up their marriage after the death of their child.

Far from any hospital, the bus pulls into a remote village where the locals try to help. The husband phones the US Embassy, which assumes an act of Muslim terrorism.

At home in San Diego their Mexican nanny, who was supposed to have time off to go to her son's wedding but is unable to find a baby-sitter, goes anyway, taking their two other children with her and driving across the border with her hot-headed nephew. In Tokyo, a deaf mute girl finds she too is connected with the crisis.

Babel, which is shot with actors and non-actors in four languages - the title is a reference to the biblical story in which God caused each person in a tower built to reach heaven to speak in different tongues - is a compelling exploration of the fear of "the other" that haunts the post-9/11 world. It draws on a paradox that while digital technology is providing the means for a cyber-democracy in which billions can directly communicate, the prejudices that separate us are ever greater. By allowing the stories to unravel out of order, Babelmirrors this sense of lost people pushed to the edge of confusion and despair as they try to find someone to trust or to make some sense of what is happening: the form of the movie brilliantly expresses its content.

Although it stars Brad Pitt and Cate Blanchett as the conflicted couple, and Gael Garcia Bernal as the nephew whose rage at American border guards imperils the nanny and two small children, Babel is an ensemble movie in which no one performance takes centre stage.

It may be shot on an epic global scale, but it draws its coherence and suspense from the intimacy and humanity of each story. Cinematographer Rodrigo Prieto finds a visual parallel to the emotional journeys of the characters through subtle shifts in colour and texture - orange earth tones in the Moroccan sequences, vivid red for Mexico, and hard-edge red-purple for Tokyo - while composer Gustavo Santaolalla's minimalist score relies on the Arab instrument the oud, an ancestor of the guitar, to provide a unifying cross-cultural rhythm and sound.

Babel is a meditation on the power of life. Far from being fatalistic or contrived, it challenges the inevitability of fate. It seeks redemption in the sheer act of making choices. "This is our only life, " Arriaga said. "This is the place where we have to do things right."

As with Kieslowski's great Three Colours trilogy, Gonzalez Inarritu and Guillermo Arriaga with Amores Perros, 21 Grams and now Babel have created a work of rare humanity, daunting in scope but compelling in its ability to touch the experience of our own lives.

In the "fragile city" of the 21st century no one can be an island on to themselves: to survive we must find it in ourselves - as the stumbling characters in Babel learn - to be open to, and trust, the "other".