Broadcaster Bill Moyers recalls that he was in a plane high above the North Atlantic, returning from the 1986 Reagan-Gorbachev summit in Iceland, when the hymn "Amazing Grace" began repeating itself in his head.

He doesn't know what triggered the connection. Something at the summit? Flying over the ocean? Something lodged in his memory that had been jogged by the stopover in London?

Four years later, the TV journalist and cultural commentator has no answers to those questions. But he has satisfied the journalist's curiosity that compelled him to jot down, during that flight, an assignment for himself: "Amazing Grace -- who wrote it and why?"

What he eventually learned about this hymn, one he had heard countless times in his Baptist church as a youngster, was fashioned into a special, "Amazing Grace with Bill Moyers," that will be aired Sept. 12 by the Public Broadcasting Service.

Early in the project, Moyers found out that, according to the Library of Congress, "Amazing Grace" had been done by more performers than any other hymn.

He spoke with folk singer Judy Collins, who popularized the hymn during the 1970s, and he learned that the song was much more than a popular part of her repertoire. "She told me 'Amazing Grace' was the only song she had been able to get through" during her battle with alcoholism, Moyers said in a recent interview.

{D.C. Mayor Marion Barry invoked lines from the hymn several times during and after his trial on cocaine and perjury charges.}

Moyers discovered that the song he had heard in black congregations throughout the South during the civil rights struggle was the work of an 18th century English slave ship captain named John Newton. By the time he wrote the hymn, Newton had become a country preacher working against the slave trade at which he once prospered.

In Moyers's special, the background of the hymn and Newton's personal history are woven through interviews and performances that begin to demonstrate the enduring power of "Amazing Grace."

In sanctuary, concert hall, prison and back yard family reunion, the images convey the force of lyrics and melody that transcend boundaries of race, denomination and social station and hit deeply in the human heart.

"Amazing Grace! How sweet the sound that saved a wretch like me! I once was lost but now am found; Was blind, but now I see."

"When I sing that song, I could be in a dungeon, or I could have chains all over me, but I'd be free as a breeze," Johnny Cash says at one point in the documentary.

Recalls folk singer Jean Ritchie at the site of a family reunion in Kentucky: "I asked my mother why these old songs were so sad and mournful-sounding . . . and she said, 'Well, honey, they're not sad, they're reverent.' She said, 'You get so much more joy out of a song when you relish the notes and savor the tune.' "

In the interview, Moyers said he witnessed the universal appeal of the hymn in January during a special showing of the program at Lincoln Center. The occasion was a benefit for The Boys Choir of Harlem, a group featured in the film, and it was attended by a wide representation of believers and nonbelievers. It was evident, said Moyers, that the song had an effect on everyone, that it had all the qualities "a really good spiritual can convey -- unity, community and transformative power."

And what would he have to say about the place of the hymn in his life?

"I remember it from my days at Central Baptist Church in Marshall {Tex.}. How sheltered and welcomed and valuable I felt. Even a little boy in East Texas can have a sense of worthlessness growing up. That hymn made me feel valuable, worthwile. The tenderest, most intimate moments of my childhood are associated with that hymn."

So too are some moments of his adult life. Moyer recounted that his father, who died last April and had not seen the documentary, requested the song be sung at his funeral.

And the lyrics of the hymn have helped him deal with the news that a friend has cancer, he said.

He sees in the song's continuing popularity its ability to "transcend the individualism and fractured condition" of contemporary society. "It rescues us from self-absorption when we need it most." There is a "gritty realism" to the film and the song, he said, that lets us know the "sense of wretchedness is not a permanent condition."

That idea is perhaps best explained by a story Moyers tells about an encouter he had when the documentary was shown earlier this year at a public theater in Austin, Tex. On opening night he was approached before the start of the program by a man who said he was a Vietnam veteran.

Afterward, the man showed up again, shaken by the program. When the theater emptied, Moyers sat with the man, who went on to tell him he had been a helicopter gunner in Vietnam. "I can't tell you how many times I sprayed villages mindlessly," he told Moyers. The man also had been a member of a reconnaissance, or "ambush," unit, as he put it. Moyers recalls the man saying, "All of the lives I took were not Viet Cong," and that he knew he had taken the lives of "a lot of innocent people."

The veteran said he had struggled with those memories for 15 years and had finally realized during the program that if John Newton could come to terms with his grief over selling people to the slave trade, then he could come to terms with the grief he felt over his activities in Vietnam.