BOSTON -- The Boston Pops will celebrate its 125th birthday Tuesday with a special world premiere -- a musical tribute to the Kennedys.

"The Dream Lives On: A Portrait of the Kennedy Brothers" is narrated by some of the greatest actors of stage and screen today.

Tuesday's final dress rehearsal was the final step in fulfilling a four-year dream of conductor Keith Lockhart. Lockhart was initially interested in a tribute to President John F. Kennedy until Gov. Deval Patrick suggested he honor all three Kennedy brothers -- John, Robert and Ted.

"Because the words of these three gentlemen -- Jack, Bobby and Ted -- together are responsible for one of the most consistent and compelling American political thought for this half-century," Lockhart said.

Their now famous words are narrated by three of today's greatest actors: Morgan Freeman portrays Ted; Ed Harris recites the words of younger brother Robert; and Robert DeNiro voices John.

DeNiro, who campaigned for President Barack Obama, alongside of Ted Kennedy, has known the family for years.

"I get emotional to this stuff, with the whole family. So, I think that says it. It's hard for me to say anything else," DeNiro said.

Rhode Island native Peter Boyer wasted no time in agreeing to compose the tribute.

"I traveled to Arlington National Cemetery to visit the graves of the Kennedy brothers. It's an overwhelming experience. And, so for me, this could be no greater honor than to be asked to do this today," Boyer said.

Award-winning lyricist Lynn Aherns worked to weaving their words into music.

"Little by little, the speeches began to surface that seemed to lead to a climax and an emotional end point," she said. "As a whole, it's extremely dramatic. It has an ebb and flow like the water they loved so much."

"What we have is the opportunity to create something that will outlast all of us. And hopefully memorialize and enhance these words even more in a way that speak more clearly to future generations," Lockhart.

Lockhart received the Kennedy's blessing for the piece, but they asked not to be involved in its creation.