BY BOB STRAUSS >FILM CRITIC

A week before Hurricane Katrina devastated New Orleans, 24-year-old aspiring rapper and Ninth Ward resident Kimberly Roberts bought a $20 camcorder on the street. She turned it on for the first time the day before the storm made landfall, then again and again as the waters rose and no help came for days, until the battery died.

Fifteen minutes of her astounding, terrifying footage anchors the documentary "Trouble the Water." But it's arguably not even the most impressive thing about this heart-rending movie.

Directors Tia Lessin and Carl Deal, both veterans of Michael Moore's producing team, use the very specific story of Kimberly and her husband, Scott, as the core of a comprehensive picture of what went wrong with the response to the disaster and how a small group of already disadvantaged people, forced to fend for themselves in the face of overwhelming hardship, faced the task as best they could.

The professional documentary team followed the Robertses and some fellow survivors north through Louisiana and to Memphis in the days and weeks after the storm, then back to New Orleans to assess the damage. Bodies of loved ones are discovered, survivors are joyously reunited with still-living dogs, and rapprochements are made with the authorities who failed to help in the time of greatest need. The difficulty of starting over again, or just getting through the day, is minutely detailed.

This would all be pretty depressing, and in fact, it is. But it would be unbearable if Kim and Scott weren't the marvelously charismatic screen presences that they are. And seeing them and some dysfunctional neighbors whom tribulation turned into an ad hoc family grow or change for the better certainly provides some glimmers of hope.

Breathtaking doesn't begin to describe the scene in which Kim finds a demo tape she made and had assumed was lost, plays it and, in the little room she's staying in, raps out her personal anthem "I'm Amazing." If that doesn't get this woman a recording and video contract, the music business is beyond repair.

Expert usage of news footage and other media - the chillingly resigned 911 calls for help that won't come are full tragedies in their own right - tell the broader story. But "Trouble the Water's" greatest of its many values is its incisive record of how a massive catastrophe is really a story of multiple individuals, families, blocks and neighborhoods. Kim's right, she is amazing. But so is this commitment to humanity.

TROUBLE THE WATER

Not rated: language

Director: Tia Lessin, Carl Deal.

Running time: 1 hr. 36 min.

Playing: Sunset 5, West Hollywood; Westpark 8, Irvine.

In a nutshell: Deeply moving Hurricane Katrina documentary includes footage taken by young rapper Kimberly Roberts while she was trapped during the storm in New Orleans.