Though it is essentially a concert film, Don’t Look Back In Anger takes two adjacent angles on the band’s story. One is their appeal; Oasis peaked in 1996/7, seeming to confirm a theory that rock and pop music worked in ten-year cycles, starting with the emergence of Elvis Presley in the late ’50s. Oasis and their rivals Blur went on to launch the second British invasion of America, but, unlike Blur, Oasis ran out of road. Sibling rivalry was the least of their worries; although the band recorded seven albums, most people only remember two, and it is these records — their 1994 breakout Definitely Maybe, and its almost immediate follow-up, (What’s the Story) Morning Glory? — that form the bulk of the music that still commands an impressive international fandom.

The first half of the movie reflects the response that came when the reunion was announced: No one could quite believe it. Like many films in the Venice film festival this year, Don’t Look Back In Anger deals with the concept of the self-made myth, and, in their heyday, an effective part of the Oasis myth is that anything could happen and usually did. Noel Gallagher rarely gets the credit for this, being a rare example of a Malcolm McLaren-style impresario who directed the band while keeping his head down, knowing that the core of the band was Liam’s ridiculous peacocking, not his melodic songs about magic pies. Oasis were always meant to be rock’n’roll, and without Liam, they didn’t have it.

The divine force that brings them together — some might say their mother Peggy, who is curiously absent, even though Noel muses on the brother’s solid Irish roots — is never quite explained, but the first scenes suggest that the foundations of the reunion were rocky from the get-go. In a vast rehearsal room that recalls the one The Beatles used to record Let it Be, Noel is uncertain whether Liam will even show. “I just don’t see it,” he says. “Will it fall apart again?” He’s not the only one: Someone taunts the band’s rhythm guitarist, Paul “Bonehead” Arthurs, missing from the band since 1999, with the same thought: “You’ve come into the middle of a Gallagher sandwich, you c*nt”.

Bonehead, it transpires, is largely what was missing from the Oasis sound that sent their sales figures plummeting; the band’s elephantine guitar sound is perfect for radio, and without him their sound diluted, as Noel acknowledges. Blur started calling them (Status) Quoasis for this very reason, and it must have hurt, mainly because it’s true. When Liam finally rocks up, and this is much the mot juste, he is in quite amazingly good voice, given the recent sorry state of their closest rivals, The Stone Roses, in particular their singer Ian Brown. He sweeps in like ghost of Christmas present and suddenly everyone — everyone being Noel — starts to wonder if this just might work after all.

The first gig in Cardiff is a blinder, notable for the fact that, despite being the audience for a 30-year-old reformed heritage act, there are people under 30 and half of the adults still seem to have their own hair. These are not your average concertgoers, says the local police chief, this is more like a football crowd, and as such their passion for the band is tribal, bordering on religious. Says one fan, in all seriousness, the reunion is “like the rebirth of Christ, or something” (it’s the “or something” that slays you).

The band’s appeal is certainly strange, given that their sound is an amalgam of punk, mod and glam with a huge chunk of The Beatles (in their prime, they looked more like The Fab Four than the cast of Sam Mendes upcoming biopic). But somehow, with the obvious exception of The Beatles, Oasis have transcended their influences, and no one in the band quite knows why: Their sometimes-nonsensical lyrics, which once suggested they’d spent too much time listening to “I Am the Walrus”, are now sung by crowds of up to 90,000 people, imbued with the same profundity afforded sacred ancient texts.

The other storyline is how the Gallaghers came back from the brink and formed a newfound bond. This is given an almost mystical significance, made all the more ridiculous when an actual priest gives a sermon using the brothers as an object lesson in the value of forgiveness. The thing that actually seems to have happened is that has finally understood that Liam, in his own way, is as self-aware as he is; giving a very funny diatribe about his stage presence, the famously static singer notes, “You’re going to be doing this till you’re 60, you don’t want to do all that Mick Jagger shit.”

At two hours, it’s a long haul if you’re not a fan, and even if you are a fan, there are lots of unnecessary chats with fans from all over that will take away from the powerful live material. As all roads must lead to Rome, it eventually leads to the band’s “Stairway to Heaven”, the sentimental song that gives the film its name and, in light of the famous phrase about familiarity, quite a lot of non-believers the heebie-jeebies. If you can grin and bear it and turn a blind eye to the repetition of scenes in which the Gallaghers reiterate their astonishment at this unexpected second act in their lives, Don’t Look Back in Anger offers some useful information to nurture everyone’s budding inner rock star. “It takes a lot of caring,” says Liam, “to not give a f*ck.”

Title: Oasis: Don’t Look Back In AngerFestival: Venice (Venice Open – Out of Competition)Director: Dylan Southern, Will LovelaceScreenwriter: Steven KnightWorld sales: DisneyRunning time: 2 hrs 2 mins

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