In one way, “Once Were Brothers: Robbie Robertson and the Band” is an odd choice to serve as the opening-night film at the 2019 Toronto International Film Festival. At a festival chock full of major awards contenders with big movie stars, it’s a documentary about a musician whose music is mostly heard on the occasional oldies station or Americana Spotify channel, by a director, Daniel Roher, with only one previous feature on his resume.

But “Once Were Brothers” sports Ron Howard, Brian Grazer and Martin Scorsese among its executive producers – and more than that, Robbie Robertson is a local hero of sorts, born and bred in Toronto before he headed to the U.S. to become an unlikely rock star.

Besides, TIFF has always had a tricky time with its opening films, most of which go on to achieve no particular success. Over the last 10 years, the slot has gone to a couple of big studio movies (“The Magnificent Seven,” “Looper”), a few movies that didn’t register commercially or critically (“Outlaw King,” “The Judge,” “Borg McEnroe,”), outright curiosities (“Score! A Hockey Musical”) and one other music documentary, Davis Guggenheim’s U2 doc “From the Sky Down.”

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“Once Were Brothers” doesn’t seem likely to change that track record much, but it’s a solid chronicle of (the first part of) a fascinating life and career. Robertson was a 15-year-old kid who managed to get a gig playing rock and rockabilly around North America; who stumbled into a musical relationship with Bob Dylan that produced some of the fiercest and most seminal moments in rock history; who helped invent the genre called Americana even though he and four-fifths of his band were Canadians.

Like Robertson’s 2016 memoir, “Testimony,” the film sticks with Robertson’s early years and with his career up to the point in 1976 when the Band called it quits with the star-studded farewell concert known as The Last Waltz.

The truncated timeline was a curious choice on the page, though Robertson had no problem filling 700 pages with those years. It’s a little more frustrating on the screen, where you see the 2019 version of Robertson talking about the first section of his life but ignoring almost everything that has happened over the last 43 years.

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It not only suggests that those decades were less interesting (which, yeah, they were), but it almost implies that he didn’t do anything noteworthy for the last four decades, which is hardly the case. Hell, he’s got a new album and a soundtrack to Martin Scorsese’s “The Irishman” coming out shortly.

But there’s plenty to relish in the story that the film does tell, from his days as a teenage whiz-kid guitar-slinger too young to get into the clubs where he played every night with rockabilly wildman Ronnie Hawkins to his 1966 tour with Bob Dylan, who antagonized uncomprehending audiences worldwide by giving his folk songs a jolt of electricity and rage.

The film skims lightly over the gang ties of Robertson’s father and uncles, and over the break with Hawkins (hint: it wasn’t as easy as it sounds here), but it has a bigger story to tell: the story of the Band, which turned from a backing group to one of the most distinctive, indelible and lasting groups of its era after workshopping in an ugly pink house in Woodstock, New York.

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Mind you, “Once Were Brothers” doesn’t really explain how they did it, how a group of blues-seeped rockers somehow tapped deeply into the myths of rural America to write and perform songs as seemingly timeless and bottomless as “Up on Cripple Creek” and “The Weight.”

Robertston takes a stab at explaining that last song, which was inspired by the “Nazareth, PA” stamp on the inside of his Martin guitar. But the mysterious eloquence of “The Weight” still resists explanation, even from the guy who wrote it.

How’d they do it? Who knows, but they did, at least twice: Once on their haunted and haunting debut album, “Music From Big Pink,” and again on the sleeker follow-up, “The Band.” As luminaries from Bruce Springsteen to Eric Clapton to Van Morrison testify in the film, Robertson and his bandmates – Levon Helm, Rick Danko, Richard Manuel and Garth Hudson — really did seem like brothers. Their musical empathy was rich and deep, and it was easy to assume that their personal empathy was, too.

And for a while, it was. But the film’s title, “Once Were Brothers,” hints at the tragedy in this story. Drugs and alcohol helped tear the group apart, and so did personal resentments over Robertson’s increasing role and increasing profile as the group’s mastermind. (He wrote many of the songs at first and almost all of the songs by the end, but the sound of the group relied as much on Helm, Manuel and Danko, its three magnificent vocalists.)

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Helm, an Arkansas native who showed the teenage Robertson the ropes in Hawkins’ band and was the closest thing he ever had to a real brother, grew particularly embittered and estranged from his longtime pal. By the time the Band ended its touring career with the Last Waltz – a lavish event and Scorsese-directed concert movie that featured way too much Robertson for the other members’ liking – the break was complete.

(It’s not mentioned in the film, but the other members got back together and recorded and toured without Robertson for years until Manuel hanged himself and Danko died of a heart attack after years of drinking and drugs.)

Those deaths, the cancer death of Helm in 2012 and the members’ estrangement from Robertson hang over “Once Were Brothers,” which is named for a new song that Robertson previews at the beginning of the film. “Once were brothers,” goes the opening line. “Brothers no more … It was so beautiful it went up in flames.”

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We see some of those flames in Roher’s film, which tells Robertson’s side of the story and makes use of what little concert footage exists of the Band in its prime, as well as in memories from people who were close to Robertson. (Helm and some others, including George Harrison, are seen in interview footage not shot specifically for this film.)

This is a movie in which the sheer joy of rock ‘n’ roll gives way to a deep sadness. Its most powerful section, in many ways, is an extended sequence that deals with the estrangement between Helm and Robertson, and that ends with the incredible performance of “The Night They Drove Old Dixie Down,” a song Robertson wrote just for Levon, at the Last Waltz. (I was there that night, and it was even more staggering in person than it feels on screen.)

At moments like that, “Once Were Brothers” – and the past tense in that title is so, so important – has the power to transport you but also to break your heart.