The heart fair sinks in the first five minutes, when you start and wonder if America's most politically committed film-maker hasn't somewhere lost the plot. It's South America, and the recently widowed Dr Fuentes (an excellent Federico Luppi) resolves to use his upcoming holiday to travel into the mountains. He wants to check on the young students he trained on an international aid programme as doctors for his country's poorest villages.

On arriving in the first he hears talk of how their medic was burned alive by "men with guns". He presses on, however, picking up a young orphan, a renegade padre, a deserter and a girl rendered mute after being raped by a band of soldiers. The deeper they go, the more the pattern repeats: all of Fuentes' former protégés have been murdered, for tending both to wounded guerrillas and injured government soldiers.

Our initial problems come from the doctor's almost farcical naiveté. He is an educated man, a man of means, and yet the city has apparently cocooned him from the military crackdown in the country beyond. His companions look to be ciphers, planted by a screenplay with an agenda.

And yet, slowly (this is a long, deliberately slow film) Men With Guns reveals its purpose. The country is never identified (Sayles researched Guatemala, shot in Mexico), to allow the story to assume mythic qualities the more it progresses from the ostensible civilisation of the city to the jungles of the interior. It's a road movie that ultimately abandons the road entirely and disappears into the clouds. Fortunately, however, it side-steps the obvious temptation of Magic Realism: it's a very hard-edged film.

Sayles has elected to write this almost entirely in Spanish, with subtitles. The only American voices belong to two gauche, liberal tourists who, it transpires, know more about Fuentes' native land than he does himself. Unlike him, though, they are only passing through.

Men With Guns is a major departure for Sayles. Not only does he shoot in a foreign tongue and fail to rely on his usual stock company, it eschews much of the realism for which he's justly celebrated in the likes of City of Hope and Matewan. Indeed, in its embodiment of myth and storytelling it has more in common with his limp kiddie-flick The Secret of Roan Innish. Here the people the doctor meets are labelled by what they harvest (Gum People, Salt People, Corn People), else they are the titular "men with guns". It's a distinct change of direction after the complex, commercially successful Lone Star. Visually, certainly, he's on a limb, mastering the jungle in the distinctive honey-rich colours of Polish cameraman Slawomir Idziak (Blue and Veronique for Kieslowski, Nicols' Gattaca). The film looks extraordinary.

Men With Guns is also arguably John Sayles' most overtly political work. It's agenda rather than character driven, and asks that it be approached more on its own terms than as some front line dispatch. It's didactic in a way that even City of Hope and Matewan are not: which, one senses, was his purpose. Sometimes, you can hear him say, it's better to tell people what you think than just show them.