A terrible thing happened in Atlanta on April 26, 1913, but it didn't end there. It didn't end, really, until 1986. Now, NBC is reopening the case again, with its two-part, five-hour movie "The Murder of Mary Phagan," airing tomorrow night at 8:30 and Tuesday night at 9 on Channel 4.

Chronicles of terrible things have made strong TV movies before, but "Mary Phagan" seems extraordinarily and thoughtfully powerful. It starts slowly and cautiously, and the production has an occasionally off-putting austerity to it, but by the end, the cumulative impact is immense.

Mary Phagan was a 13-year-old girl who was killed at the pencil factory in which she worked while fellow townsfolk celebrated a Confederate holiday. Charged with the crime was Leo M. Frank, manager of the factory. An outsider, a northerner and a Jew, Frank became a scapegoat for regional, social and ethnic hatreds of the time.

He was made a symbol, too, of economic hardships brought on by urbanization and the Industrial Revolution. What nobody then called "the media" -- the newspapers -- crucified him with anti-Semitic invective.

Frank was convicted on flimsy evidence during a trial in which the chants and threats of a mob (including "Hang the Jew") could be heard inside the courtroom. After reviewing the testimony, Georgia's governor, John M. Slaton, commuted Frank's sentence from death to life imprisonment. But another mob dragged Frank from a jail and hanged him from an oak tree near Mary Phagan's birthplace.

An ugly story, a shameful story, inflammatory for decades. Why bring it up again now? The film justifies itself every step of the way, but especially in its second half, when the script concentrates on the ordeal that faced Slaton, who threw away a political career in his attempt to prevent injustice.

Jack Lemmon plays Slaton in the film and does a tremendous job of it. The nervous mannerisms one associates with Lemmon are held in check; the presence of star power and the authority the veteran actor brings to the role are great assets. This is a stirring, heartening portrait of monumentally stubborn integrity, and as Slaton painstakingly goes over details of the trial, the film is persuasively suspenseful besides.

Slaton, as the film tells it, had virtually no encouragement but his own ideals. Political bosses coerced him. The press vilified him. Torch-carrying crowds besieged him. He persisted. In a speech he gives upon leaving office, movingly delivered by Lemmon, he explains why he commuted the sentence over deafening public outcry: "I cannot abide the companionship of an accusing conscience."

Just about everyone in the cast rises to the high standards of the production, including Robert Prosky as a definitively despicable politico and Richard Jordan, who's found a new career playing devious heavies, as the blindly ambitious solicitor general. Kathryn Walker plays the governor's wife, cunningly supportive, and Wendy Cooke has the brief but pivotal role of Mary.

Even with so much good acting going on, Paul Dooley stands out, in Part 2, as William J. Burns, a crack detective who later ran the FBI. But the most difficult role went to Charles S. Dutton, who plays Jim Conley, a poor, alcoholic janitor who is the state's chief witness against Frank and who, it is strongly suggested, actually committed the crime.

Dutton is mesmerizing in scene after scene, and it can't have been easy for a black actor in the self-conscious 1980s to play a character like this, who grovelingly addresses all white men as "boss." Never mentioned in the screenplay is the obvious point that if the finger of guilt had pointed at Conley in the first place, there would also have been a lynching -- probably without the nicety of a trial beforehand.

One weak spot in the cast is Rebecca Miller, the daughter of playwright Arthur Miller, here making her film debut. As Frank's wife Lucile, Miller is so vague and muted that she makes the woman utterly passionless. Peter Gallagher, as Leo Frank, probably overdoes the underplaying, too, but at least he seems more haunted than vacant.

The Frank case has been dramatized before, most notably in "They Won't Forget," directed by the late Mervyn LeRoy in 1937. The film was based on a novel, "Death in the Deep South," that was in turn based on the Frank case, though with some of the more volatile social issues diluted.

"They Won't Forget" is mostly remembered now as the film that introduced Lana Turner, in the role of the murdered girl. LeRoy's movie aimed to make an audience's blood boil, and it remains effective at that even now. But the filmmakers behind "Mary Phagan" avoid trafficking in the kind of hysterics that surrounded the case in the first place.

In that pursuit, they have toned down some of the details -- even the horror of the lynching itself, which is depicted as having been an occasion of heads-bowed chagrin on the part of the lynchers. According to observers, they shouted anti-Semitic epithets.

But these and other choices made by Jeffrey Lane and George Stevens Jr., who wrote the script from a story by Larry McMurtry, seem sensible and dramatically valid. Billy Hale, who directed, used a formally deliberate approach, giving the film a dark-spirited folk-tale quality. It is hard to recall a better looking TV movie. Richmond does a particularly telegenic job of substituting for the Atlanta of seven decades ago.

The voice of Stevens, who also produced, can be heard at the conclusion describing the aftermath. The lynching of Frank is considered the most violent act of anti-Semitism in American history; no one involved was ever prosecuted. There was also a wave of violence against Jewish businesses in Atlanta when the sentence was commuted.

In 1986, the Georgia Board of Pardons and Paroles granted a posthumous pardon to Leo Frank, who is buried in his native Brooklyn. Stevens doesn't mention that three years earlier the board refused to grant the pardon. Even in reversing itself, it declined to absolve Frank of the crime. The pardon was granted because the state of Georgia had failed to provide for his safety while in custody.

"The Murder of Mary Phagan," by bringing the case to national attention in the way that only network television could do, is in its way an extension of Leo Frank's pardon, a belated act of justice. Just as importantly, in recalling the sacrificial triumph of John Slaton, it's a valuable and memorable profile in courage.