What if the slacker icons of American cinema lapsed into middle-age mediocrity? Ferris Bueller doomed to forever repeat high school, Wayne and Garth working dead-end retail jobs at Guitar Center, the Teenage Mutant Ninja Turtles gone to seed in some depressing backwater freakshow?

Arriving three decades after time-travelling teen trip Bill and Ted's Excellent Adventure (1989) and the Bergman-meets-MTV Headbanger's Ball that was Bill and Ted's Bogus Journey (1991) — two blissfully dopey hangouts that helped launch Keanu Reeves to stardom and enshrine dudespeak in the pop pantheon — Bill and Ted Face the Music might be the first American film of its kind to revisit its heroes as they enter most unprecedented middle-age, like some mournful air-guitar riff on Michael Apted's Up series.

No way? Yes way!

When we last encountered Bill S. Preston, Esq. (Alex Winter) and Theodore "Ted" Logan (Reeves), the Great Ones were on their way to rock 'n' roll superstardom — and brokering world peace — with their triumphant metal band, Wyld Stallyns.

According to prophecy, they would write a smash song and usher in a utopian future so bright that everyone — including pan-dimensional gurus who morphed from George Carlin to Pam Grier — was literally wearing shades.

Only, the prophecy was never fulfilled.

Welcome to the most non-non-heinous present

Yet to write the song that unites the world, Bill and Ted are back moping around the vanilla suburbia of San Dimas, California, playing to ever-diminishing audiences, and attending couples therapy with their erstwhile medieval princesses, the impossibly patient Elizabeth (Erinn Hayes) and Joanna (Jayma Mays).

"I'm tired, dude," sighs Ted, and Reeves's rusty delivery — a dissonant side effect, perhaps, of an adult career in the post-human sublime — certainly reflects it. (A wide-eyed Winter, on the other hand, bounces back into character like he'd just stepped off the set of the previous instalment.)

At least their genius-doofus daughters — Wilhelmina "Billie" Logan (Brigette Lundy-Paine) and Theadara "Thea" Preston (Samara Weaving), named in amusing homage to their dads' bromance — have inherited their once-boundless spirit and capacity for accidental enlightenment.

"The wedding song was most luminous," Billie tells them, after Ted's avant-garde theremin noodling bewilders the guests at a family wedding. "Your playing rivalled Clara Rockmore," Thea adds.

With Bill and Ted going nowhere, the fabric of space and time has begun to unravel. In a series of irreverent visual gags that feel amusingly of a piece with recent cultural discourse, history as we know it is being turned inside out: rappers are dropping in on the Last Supper, the Pyramids are materialising in Southern California, and the original films' funky, physical future has been replaced by charmless green-screen vistas.

Whisked back to the 28th century by a concerned future emissary, Kelly — Kristen Schaal, playing the daughter of Carlin's righteous mentor, Rufus — our favourite airheads are told they have until the end of the day to write the song that will unite the world and "save reality as we know it".

The ultimatum compels Bill and Ted to — if not exactly write the song — then attempt to steal it from their successful future selves.

It's a quest that sends them pinballing across the space-time continuum to collide with various "dickweed" doppelgangers, including a pair of greasy bar-room burnouts, two impossibly 'roided-up macho men, and some spandex-clad Spinal Tap geezers with suspiciously dodgy accents. (What joy it is to hear Reeves revive his camp English pronunciation, perhaps the sweetest ASMR sound in the world.)

Meanwhile, Billie and Thea embark on their own excellent adventure to assemble a super-group to help write the tune — collecting the likes of Jimi Hendrix, Louis Armstrong, Mozart, legendary Chinese musician Ling Lun, and a cavegirl drummer named Grom along the way.

Also hitching a ride: guest star Kid Cudi, who proves to be an unlikely, and very useful, expert in quantum physics.

Director Dean Parisot, best known for the clever sci-fi comedy Galaxy Quest (1999), keeps the multidimensional mayhem zipping along with a light, if formally anonymous touch, even as the script — by returning series' creators Ed Solomon and Chris Matheson — can't quite recapture the haphazard brilliance of the originals, which tended to discard the boring stuff like logic in favour of making things up as they went along.

After all these years in development — and the go-for-broke weirdness of Bogus Journey, which ended with Bill and Ted playing on Mars — you'd be forgiven for wondering whether the creative team might've conjured up something more outlandish.

Yet what the film lacks in inspiration it eventually makes up for in sheer, accumulated goodwill.

Bill and Ted Face the Music is nothing if not true to the generous, guileless spirit of its predecessors, which were always about mixing high and low culture to show that real wisdom — or "knowing nothing", per So-crates — sprang from the unlikeliest places, a warm, anti-elitist sentiment that continues to flourish here.

By the time Bill and Ted reunite with their estranged former bandmate Death (an ever-wonderful William Sadler), who hasn't aged a day over infinity, and scoop up some winning new characters like Dennis Caleb McCoy (Anthony Carrigan), a killer robot with an identity crisis, it's hard not to be swept along by the obvious goofy love put into all of it.

There's the odd refreshing ripple, too, like the selfless notion this time around that it's not just about Bill and Ted, that maybe the Wyld Stallyns are but a vessel through which music might pass on to the next generation.

And Billie and Thea feel less like gender-flipped versions of their dads than fluid extensions of them — non-binary actor Lundy-Paine, in particular, offers such an uncanny, gender-ambivalent mimesis of Reeves that it transcends even his own youthful androgyny.

If the much-anticipated song inevitably disappoints — it's a cacophonous, New Age Franken-jam designed to sound less like a killer hook than an anthemic stadium chant — then it's the movie's way of finally saying it's less about the tune than those playing it; the dust in the proverbial wind, as it were.

In these moments, Bill and Ted Face the Music is almost touching, at its best without having to resort to cheap nostalgia.

"There are infinite pasts, and each is entangled with infinite futures," says an elderly, grave-bound Ted at one point, a scraggly-haired Reeves wheezing under layers of comically prosthetic makeup.

It's a grace note that's come to characterise not just the series, but the defiant will of an American pop cinema that continues to loop back on itself; the illusion of youth forever regenerating.

"In case you were wondering," explains Ted, the eternally cosmic Reeves, as he skitters across time and space during the film's climax, "I'm essentially an infinite being".

Bill and Ted Face the Music is in cinemas from September 10th.

Below : Bill and Ted's daughters Thea and Billie are respectively played by Australian actor Samara Weaving (right) and Brigette Lundy-Paine (left). Rapper Kid Cudi (middle) plays a fictional version of himself and was offered the role after the film's writers found out he was a Bill and Ted superfan.