Americans could be forgiven for forgetting about Katrina — after all, when’s the last time you heard either Barack Obama or John McCain mention it?

“Trouble The Water,” then, could not be timelier. It is a bracing documentary about Katrina and its effects on an extraordinary group of New Orleans residents. It is also a welcome indictment of the attitude of the United States toward its poor and its people of color.

The filmmakers behind “Trouble,” Tia Lessin and Carl Deal, use some stock footage, but they rely mostly on the previously untold story of Kimberly Roberts, a woman living in New Orleans’ Lower Ninth Ward with her husband, Scott. Kimberly’s a star. She has a hugely charismatic personality and seems to know everyone in her all-black neighborhood. When we first meet her, she’s behind a camcorder, recording footage of the ward and the people in it — people who, like her, don’t have the means to leave the city. In her words, she’s “showing the world that we did have a world” before the hurricane hit.

Watching Kimberly’s footage is perhaps the closest you could come to experiencing Katrina firsthand. Through her lens, we see everything: first the wind, and then the rain. We see the water tearing down the street, almost engulfing houses. We hear the call they make to 911 (“There is no rescue team,” they are told). Taken as a whole, Kimberly’s shots are the most effective condemnation of the Bush administration’s mishandling of Katrina that I have seen. But the most damning moment of all happens off-camera. After managing to escape the city, Kimberly and others retreat to an evacuated Navy base, only to be turned away at gunpoint by the soldiers, who refuse to let them stay even one night. They eventually find safety at a Red Cross shelter.

In looking at the aftermath of Katrina, “Trouble The Water” also serves as a reminder of the endemic poverty in the United States. It might have taken a hurricane for the media to remember that there were poor people in this country, but the residents of the Lower Ninth Ward need no reminder. One talks of “the hurricane we had before the hurricane hit.”

Despite the film’s grimness, a hopeful spirit manages to emerge in the end. This is the good side of humanity: people risking everything to help others whom they might not even know, never giving in to the horror surrounding them. Kimberly and Scott use Katrina to escape their troubled pasts. Eventually they resettle in New Orleans, where Scott gets a construction job and Kimberly works on her fledgling rap career. She’s good, actually.

Lessin and Deal make sure that their film is not just a feel-good story, but one with a sharp political edge. So we get the woman who forbids her son from serving in the army for a country that won’t fight for him, and Kimberly’s cousin in Memphis, who notes with despair, “If you don’t have money and you don’t have status, you don’t have a government.” Ultimately, though, the power of Kimberly Roberts’ personality and story lifts “Trouble The Water” from a polemic to a tale of human tragedy and redemption.

See this movie and get angry.