THERE is a moment in Black Gold, a documentary film about the global coffee trade, when an Ethiopian fairtrade activist called Tadesse Meskela addresses a gathering of coffee farmers. They stand with eager faces and Sunday-best jackets on. They are farmers from Oromia, a region that produces world-beating coffee beans, but the men are destitute: since the collapse of the price of coffee in the early 2000s, famine threatens to consume their lives. Meskela asks if they can guess how much a kilo of coffee costs in the west. But they look bewildered. He tells them a kilo will fetch $230. "And how much do you get for a kilo here?" says Meskela. They look about themselves for an answer. "Two birr ($0.23 cents) if we're lucky, " one says. And you can see their faces darken in despair at the realisation.

This documentary film by English brothers Nick and Mark Francis hits the spot like high-grade espresso: undiluted, powerful, it produces a heady spin that alerts your concentration. I went into it with an overpriced latte in my hand having not given a thought as to the coffee's origins. After 78 minutes I was stirred, vowing never again to buy coffee that was not fairtrade. Job done then for the Francis brothers.

Black Gold is a political documentary, or advocacy film, of the kind that has exploded in cinemas in the past few years since distributors realised documentaries could make money (we have Michael Moore to thank for that). Cheap to make and earnest in personality, many of them have production values that grate the eyeballs and hector the viewer like a bullhorn. Often, it's like being cornered in a pub.

Black Gold certainly looks lo-fi and it's a po-faced affair. But it sets itself apart by adopting a tone that eschews browbeating. It is so convinced of its points that the viewer does not have to be goaded into drawing conclusions. It lets the camera tell the story.

The directors achieve this by simple contrast: counterpointing coffee drinking in the west, where two billion cups of coffee are consumed daily, with coffee farming in Ethiopia. We visit champion coffee makers at the world barista championships; there is a trip to the trading floor of the commodities exchange which controls the price of coffee; and there is a spectacularly cringeworthy moment when a brainwashed Starbucks manager in Seattle gushes about all the lives she is touching by selling coffee.

But then we visit Ethiopia: the women who, in nonstop eight-hour shifts, pick out bad coffee beans by hand for half a dollar a day; the farmers who live with their extended families, 15 to a hut in poverty.

Since the collapse of the price of coffee, many are now uprooting their coffee crop and growing a leafy narcotic called chat . . .the price it fetches puts food on the table.

Tadesse Meskela emerges as a modern hero. He is the founder of a union representing 74,000 coffee farmers and deals directly with the west, cutting out the middle-men to get the farmers a fairtrade price for their coffee. Profits are reinvested in the local communities so that schools can be built. At a meeting where farmers agree to allocate profits to the building of a new school, one farmer chips in: "Even if I don't have any money, I can sell my shirt and give the money so that my children can learn and for my country to grow."

Meskela has a tough time. After oil, coffee is the most traded commodity in the world but the prices are controlled by four multinationals: Kraft, Nestle, Proctor & Gamble and Sara Lee (none of which, along with Starbucks, would speak to the filmmakers). Farmers make about three cents for every cup sold in the west.

The film confronts a paradox: Ethiopia is on the brink of famine and continues to receive aid. But the country has a crop that is in huge demand. The west, however, contributes to keeping prices low, making farmers bankrupt, while it dumps its own subsidised farm produce on Africa in the form of aid.

Globalisation here is shown to work in one direction and the film exposes a hypocrisy at the heart of the west's attitude to Africa. It's the rare kind of film that has the power to change lives.