The world may be exhausted by celebrity charity drives, but the sobering, irritating fact behind their continued existence is, they work. At least, they do in their first objective - to attract attention. Chances are, for instance, that if Natalie Portman had not visited Uganda with a camera crew, the story of Nicholas - an HIV-positive 17-year-old Aids orphan bringing up his sister and three brothers alone - would never have been heard.

Why should it have done? In Uganda, where 1m people live with Aids, and 1m children have lost their parents to the disease, Nicholas’s story is not only individually heartbreaking but heartbreakingly quotidian. Moreover, Nicholas is out of reach - he receives no help from local charity projects, which lack the cash and infrastructure to improve his life.

Portman visited Nicholas in her role as an ambassador for the Listen campaign, a new multimedia charity initiative designed to highlight the plight of poor children all around the world. Listen believes that using film stars to record the stories of children living in deprivation, and turning those experiences into documentaries, will prove powerful enough to persuade people to donate money to charity projects around the world. The scheme hopes to raise $1 billion over 10 years.

“I understand that people might be jaded,” Portman says. “I know I get turned off by most celebrity joining-in efforts. But I’m interested in this because it’s about diverting some of the unwarranted attention we have in our celebrity-crazed culture and passing it on to something else. We can give people a voice who would not normally have one.”

The impresario behind the Listen campaign is Tony Hollingsworth, whose previous credits include organising the Nelson Mandela freedom concert at Wembley. His plan for the campaign (see www.listencampaign.com) is complex - stringent accountability to donors, including exact details of the projects to be assisted, and a carefully staged assault on the media before two television events next summer.

Portman was chosen for the campaign - along with Samuel L Jackson and Goldie Hawn, among others - not only because she is young and pretty and recognisable but because she is not coming to these issues as a novice. Portman majored in psychology at Harvard while continuing her work as an actress and has already done some advocacy work for FINCA, the microfinance organisation, or “the World Bank for the Poor”.

Indeed, the actress was able to witness at first hand how a FINCA loan had transformed the lives of one family in Uganda. Sandra and Marvin are a brother and sister whose father left them when their mother tested HIV-positive. The children were rescued by a woman named Jacqueline Namutebi - a stranger - at a village funeral, who took them back to Kampala.

Namutebi, whose husband had died from Aids, and who was already caring for seven grandchildren, three of whom were HIV-positive, took in Sandra and Marvin and enrolled them in the local school. Through loans from FINCA, she was able to keep her small business - a shop and bar - running, and keep this gaggle of survivors together.

“That’s a very positive story,” says Hollingsworth, who accompanied Portman on her trip, “and one which shows that there are good projects doing good things. But the point about Listen is not to help the children who have already been helped, but to get to those who haven’t.”

The stories of those who haven’t, children such as Nicholas, proved almost too much to bear for Portman. “The thing that upset me most”, she says, “is that we weren’t taught about this stuff at school. We weren’t taught that half the world lives on less than $3 a day.”

Really? Portman not only attended an Ivy League university but has a family with a history of social and political activism. Her grandfather, for instance, was a socialist economics professor of the developing world who helped to set up one of Israel’s first kibbutzes. If Portman doesn’t know about “this stuff” then America has a problem.

“You know, part of the problem is, it’s very hard to imagine what life is like in a place like Uganda, just by seeing it on the TV,” says Portman. “You don’t really understand it until you see it.”

Hollingsworth’s desire, then, to make 500m people a year aware of the plight of poor children around the world is admirable. He doesn’t like to say his work is “educational” = he balks at appearing didactic - but he does recognise there is work to be done, particularly in America, where foreign stories have dropped down the news agenda.

“It is possible to watch a news programme in America - of 30 minutes or so - and think America is the only country that exists in the world,” he says. “If George W Bush travels somewhere they will cover that, but otherwise it’s all America. That’s a narrow, myopic view.”

Perhaps that explains Portman’s strange position. She is intelligent, no doubt. She is also politically engaged - she campaigned for John Kerry in the 2004 presidential election - and can talk in advanced terms about microfinance. But she was deeply affected by the grinding reality of what she saw in Africa. Indeed she revealed that, for all her academic savvy, she had a sheltered upbringing - one in which she was not allowed to walk down the street on her own until she was 18. Now, she says, she would consider ditching acting for work in the charity sector.

“I would definitely consider it,” she says. “It’s very meaningful to me. I’ve seen the power of the NGOs [nongovernmental organisations] to effect change.”

The unfortunate truth is that, for now, Portman has most impact as the “actor/famous person” she has been since she first made doe eyes, aged 13, at Jean Reno in Luc Besson’s Léon. When the documentary Why Listen? airs in June, viewers will not be tuning in to see impoverished teenage Ugandans. She and Hollingsworth both hope, however, that those are the people they will remember.

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