There is a deadeningly repetitive quality to the latest film from the indomitable English documentarian Kim Longinotto. It's hard to see how things could be otherwise. Longinotto has gone among the misused sex workers of Chicago and discovered that too many have the same story to tell. Almost all seem to have been abused – the old-school word "molested" keeps cropping up – as children and repeatedly betrayed as adults. A teenage girl, deemed "at risk" by her school, tells us how she "don't trust no man at all". If Dreamcatcher is any measure of reality, hers is surely a very sound approach.

Yet, against the odds, Longinotto’s film has good news to relate. The creator of moving documentaries on female genital mutilation and divorce in Iran has focussed her attention on an extraordinary woman named Brenda Myers-Powell. This Illinoisian dynamo is not just a person of courage and decency; she exhibits just the class of evangelical charisma you need to change the world and (less important, but relevant in this context) help a documentary come alive. We first meet her telling the story of a young sex worker who, after endless volleys of abuse, somehow managed to extricate herself from “the life” and work to help those still trapped in the cycle. It is her own history.

Longinotto throws no gimmicks at the film. There is no voiceover. A few shots of Chicago's throbbing, money-stoked skyline aside, Dreamcatcher comprises little else but verité footage of Brenda and the women she seeks to help. There is impressive resilience and surprising humour on display here. We are reminded of unhappy truths about capitalism and the US way of justice. But we also get to savour that nation's perennially stirring belief in reinvention.

Longinotto remains a treasure.