Bus 174 opens with a helicopter shot giving a panoramic tour of Rio de Janeiro, from the beachfront mansions of the wealthy up to the lush forest on top of Vidigal hill, and then swooping down into the enormous Rocinha slum. It's a literal and symbolic overview, conveying a powerful sense of Brazil's social divide. This stunning opening is just one of many memorable images in this fascinating, award-winning documentary.

The film tells two parallel stories. The first concerns a bus hijack that took place in the centre of Rio in June 2000. Because of the location and poor handling by the police, television cameras were allowed to get unusually close to the action. As the hijack became a siege with the hijacker taking the passengers hostage at gunpoint, it quickly developed into  a television sensation, bringing the country to a halt, and generating the highest ratings of the year.

The second story is that of the hijacker himself, Sandro do Nascimento, which the filmmakers piece together through interviews with participants in the siege and Sandro's friends and family. The director, José Padilha, investigates why the hijack took place, and how the example of Sandro is indicative of the broader social problems in Brazil. We learn that Sandro was just a child when his mother was murdered in front of him, leaving him destitute and traumatised, like hundreds of thousands of other street children in Brazil.

Homelessness is terrifying enough, but to understand the plight of the streetkids, we must factor in the total lack of prospects or state support, the near impossibility of finding a job, and the routine beatings and worse from the police. Astonishingly, Sandro had also been part of one of the last national scandals in Brazil, the Candelária massacre of 1993, when police killed dozens of streetkids with machine guns, following an altercation earlier in the day. Sixty-two of the group survived that night -- we're told that 39 of these had since been murdered by the time of filming.

The film argues, however, that the invisibility of these children is just as damaging as the poverty and physical danger. They form an unseen and ignored section of Brazilian society, and we feel this alienation both in the street jugglers performing at traffic lights and by watching the criminalised teenagers concealing their faces from the camera. In one disturbing and effective sequence, we see the horrendous, hothouse conditions in one of Rio's juvenile prisons, where children are routinely held for months without trial. Padilha shoots this segment in negative sepia, making the children look like ghosts, or monsters.

Importantly, Padilha is obviously aware of the dramatic potential of this story. He builds up the tension as the hijack reaches its climax with a shocking resolution. Bus 174 is a powerful and moving film -- a worthy addition to the recent canon of excellent theatrical documentaries.

(Released by ThinkFilm Inc.; not rated by MPAA.)