One of the great rock movies, “The Last Waltz” documented the all-star farewell concert of the Band, a monumental Americana group.

If that 1978 Martin Scorsese film captured the Band’s end, the new documentary “Once Were Brothers” explores the beginning. Or more precisely, the story of Robbie Robertson, the Band’s articulate and charismatic leader.

Essentially a visual version of his 2016 memoir “Testimony,” the film amounts to Robertson’s take on the truth (he served as an executive producer). And therein lies a problem.

After detailing Robertson’s fascinating background as the only child of a professional gambler and a Native American woman who raised him on the Six Nations Reserve outside Toronto, “Once Were Brothers” tells how the Band came together, thrived and fell apart.

This may be the rare music documentary that deals with drug and alcohol abuse honestly and in depth. Robertson, the Band’s songwriter, was apparently the only clean member of the quintet. Levon Helm, Rick Danko and Richard Manuel, who sang the songs, had substance issues, and they’re no longer alive to tell their sides of the stories.

Helm’s 1993 autobiography “This Wheel’s on Fire” suggested that Robertson essentially hijacked the Band, taking full credit for songs to which Helm and others contributed, and overtaking “The Last Waltz” during the editing process. The acrimony was never resolved.

Hence, we have “Once Were Brothers” — the title of a 2019 Robertson song and this movie, in which Robertson relives going to visit a comatose Helm on his deathbed to make brotherly peace.

In addition to extensive comments from Robertson, director Daniel Roher relies on archival interviews with other Band members and recent remarks from Robertson’s ex-wife, Dominique, who was there all along and is now an addiction counselor. The only other surviving member of the Band, reclusive keyboardist Garth Hudson, is not interviewed anew.

Balance notwithstanding, “Once Were Brothers” offers some fabulous vintage footage — both on- and offstage — and current testimony from Bob Dylan, Bruce Springsteen, Van Morrison, Eric Clapton and Ronnie Hawkins, the singer who originally hired Robertson, Helm and the rest as his backing band.

Music fans will appreciate this cinematic ode to the greatness of the Band, if not its lack of fairness.