Sometimes the most powerful moments in history aren't recorded on the front pages of newspapers or turned into the lead items of television newscasts.

Forty years ago today, Robert F. Kennedy gave a speech in Indianapolis that garnered little media coverage. A few thousand people showed up at 17th Street and Broadway to greet the senator, who was running for the Democratic presidential nomination. It was expected to be standard campaign politicking, much like we're seeing today with Hillary Clinton and Barack Obama.

But shortly after stepping off an airplane, Kennedy learned that the Rev. Martin Luther King Jr. had been slain. As chronicled in the powerful new documentary, "A Ripple of Hope," Kennedy's advisers and local officials urged him to cancel, worried about an outbreak of violence. In fact, 76 cities suffered riots in the aftermath of King's assassination.

But not Indianapolis.

Many people believe this city remained calm because Kennedy spoke that night. Through archive footage and interviews with people who were there, "Ripple" director/producer Donald Boggs captures an unheralded moment and crystallizes it in our memory.

Some black militants had shown up, carrying weapons and ready to fight, witnesses say. But most of the crowd learned about King's death for the first time from Kennedy's lips. After he spoke, they dispersed peacefully.

As shown in footage, it's a stunning moment. The raucous crowd falls virtually silent. Kennedy tells the black members of the audience they would be justified in feeling rage and a desire for revenge.

"Or," he said, "we can make an effort, as Martin Luther King did, to understand and to comprehend, and replace that violence, that stain of bloodshed that has spread across our land, with an effort to understand, compassion and love."

Kennedy told the crowd he could understand their loss, since "a member of my family" had been killed. It was the first time he had spoken publicly about the assassination of his brother, President John F. Kennedy.

Boggs includes detailed commentary from locals who attended the speech, including Darlene Howard, Jim Trulock, Hurley Goodall and Vechel Rhodes Sr. They help frame how this speech, virtually ignored then, had a momentous impact on Indianapolis and on their own lives.

"Ripple" is also sprinkled with details that add texture to the story -- how Kennedy, who would himself be assassinated only a few weeks later, had only one security man to protect him that day. Or how he waved off a speech his staff had written for him. Or that RFK was wearing his brother's old overcoat that night.

Today the "Landmark for Peace" rests on the spot where Kennedy told Indianapolis about the death of King, showing the two men reaching out toward one another. It's an apt metaphor for the undying dream they represent. Now there is "A Ripple of Hope" to tell the world about it.