The situation in South Africa has been explosive for so long that its explosiveness may be taken for granted. Tonight at 10 on Channel 7, an ABC News "Closeup" documentary, "Adapt or Die," looks at South Africa's struggling black trade union movement within the wider context of apartheid and finds "deep change," violent or nonviolent, inevitable.

ABC has been making promotional hay out of the fact that even after a year's worth of negotiations to get official permission from the South African government to shoot the documentary there, some 12,000 feet of exposed film were subsequently "sabotaged"--"intentionally fogged by exposure to light"--by some mysterious somebody. A few scenes in the documentary, mostly those shot at labor union rallies, thus suffer from red shadow-fringes and light bursts. This is a badge of honor and ABC News has every right to flaunt it proudly.

The film investigates growing black restlessness and tentative union organizing in such South African industries as the gold trade, which can bring in $7 billion a year and which employs 500,000 men, 90 percent of them black. They are "virtually captive workers . . . with no rights," says writer-correspondent Marshall Frady. Later, a group of dockworkers, also trying to organize, is asked, "How many of you have been harassed by the police?" and most of the men in the room raise their hands.

Ford Motor Co. is credited with being "progressive" in its policies at the Ford plants in South Africa, but a film of an auto workers' union meeting is among those fogged, and an interview with a labor leader was fogged so badly it had to be reshot.

Black workers talk to Frady about the persecution of union organizers by the police--merciless interrogations, torture, general harassment, and the most dramatic case, that of Neil Aggett, the white leader of a black trade union who hanged himself in his jail cell last year. A cellmate says Aggett was tortured while in custody.

The conditions here seem ripe for, among other things, a Costa-Gavras movie, the "Z" or "Missing," or both, of South Africa.

While strikingly well-photographed and edited, "Adapt or Die" might be more effective if the proverbial "issues" had been more personalized, with one or two case studies of union organizers covered in detail--in, as it were, true "closeup." Also, Frady's way with narration still seems mannered and too self-consciously "urgent," and when he is on camera, he appears to be doing a much over-coached imitation of Bill Moyers, but without Moyers' easy authority.

Still, "Adapt or Die," produced and co-written by Christopher Isham, succeeds dramatically at communicating the tension of, to use a key journalistic cliche', "An Uneasy Calm"--one that may be shattered at any uneasy moment.