Imagine there's no Beatles. It's easy if you try.
Easier still if the award-winning writer/director team of Richard Curtis and Danny Boyle come together to do the bulk of the hypothesising for you, as they have in their new film, Yesterday.
The movie, which premiered on Tuesday, stars former EastEnders actor Himesh Patel as a struggling singer-songwriter from Clacton-on-Sea.
His character Jack Malik wakes up after an accident caused by a global power outage to a world that has forgotten all about John, Paul, George and Ringo.
The first belly laugh drawn from this strange situation is that a cursory search online for "Oasis" no longer throws up the Beatles-borrowing Mancunian rock 'n' roll band, but merely a description of "an isolated area of vegetation in a desert".
"I can't tell you how badly that joke goes down in America," Curtis tells the BBC. "The Americans just completely assumed that Danny and I would cut it but we were having none of it. Americans don't cut jokes out of their films because we don't know where New Jersey is so why should we cut the Oasis joke?"
In this alternative Fab Four-free universe, Jack revises the lost Beatles back catalogue and claims it as his own work on a helter skelter ride to fast fame. He gets by with a little help from one of Curtis's real-life pop star friends, fledgling actor Ed Sheeran.
"When we came up to Suffolk to research the movie I just said to Ed 'why don't you come to dinner with Danny'," says Curtis, who was also responsible for Love Actually and Four Weddings and a Funeral. "I said after the dinner, 'Danny, if we cast him, he's not experienced at acting' and Danny said 'from the way he is as a person, I'm absolutely sure it's gonna be fine.'
"So Ed committed to rehearsal, instead of saying 'I'll just turn up', and Danny just gave him a few simple things. We were part of the miseducation of Ed Sheeran."
Sheeran's previous main acting role involved a brief cameo as a singing soldier in Game of Thrones and so the director had to help him navigate the major pitfalls that singers often find on the road to Hollywood.
"Obviously Ed's a wonderful musician," explains Boyle, from the adjacent room of a Soho hotel at the film's big press day, "and it was amazing watching him in concert. But actually in terms of acting I felt I could help him a bit."
Boyle advised Sheeran to "listen to the scene", rather than just learning his lines and waiting for his cue, which he says is always "the big mistake" non-actors make when performing a cameo.
"What you've got to do is listen to the actors and then it doesn't matter if you miss your cue because it's film - it's not like theatre and you can stop and go back and do it again."
While Sheeran was learning something about acting from the Trainspotting and Slumdog Millionaire director, Patel was spending more than a day in the life of a real musician. Boyle remembers "a wonderful moment" when Patel had to actually perform the "intimidating" piano ballad, The Long and Winding Road, in rehearsal for the first time and realised the Ed-ephant in the room.
"The biggest pop star in the world is listening to him do a cover of a Beatles song!
"Ed said to him 'that was very good', and it was genuine as well, he wasn't just buttering him up and I think that was a big step in his confidence level. There's something familiar-yet-strange about his performance of the songs that kind of transcends karaoke and you don't really think about The Beatles - you think about the song being re-discovered or re-presented to you."
Patel admits it was "surreal" and "a real joy" for them to find their own way into the band's famous songbook. He describes the process as "paying tribute to them, but not being beholden to them and respectfully making our own versions".
'Mega British Asian pop star'
The legendary Scousers made some major breakthroughs following a trip to India, but only a few UK artists of Asian descent have ever truly troubled the mainstream charts. Patel's character embodies the potential for a British-Asian pop giant that - namesake Zayn Malik or M.I.A aside - has been largely missing.
"I know that will mean something to a lot of people," he says. "It does break a boundary, absolutely. I'm hopeful that perceptions are changing and we move forward."
Although his newfound rock star status may be fictional, the actor and musician actually appeared on-stage at one of Sheeran's real-life Wembley Stadium gigs during filming last summer, as well as a much smaller gig at Latitude festival. He also played a rocked up guitar version of Help! in front of 5,000 people from the roof of a hotel in Gorleston during the making of Curtis' favourite scene. The writer saw the wider social effects of Patel's performance unfold before his eyes.
Curtis recalls: "The TV presenter Anita Rani came along on the day when we doing the Pier Hotel big concert and I noticed she'd started to cry watching the monitor. She said 'I've waited my whole life for a mega [British] Asian pop star and you've made us one'.
"He was cast because he was best but I love the fact that made him even more ordinary and more of a long shot. But look," he adds with a grin, "if Ed Sheeran can be a pop star anyone can!"
If Jack's Beatles-inspired chart heist is the backbeat of the film then the real rhythm comes through his relationship with childhood friend, manager and number one fan Ellie Appleton, played by Lily James. She has to look on from a distance as fame and his new US representative Debra Hamer, played by Kate McKinnon, begin to take hold.
"He doesn't just have imposter syndrome, he is a fraud - it's not his music," says James, sipping her umpteenth green tea of the press day.
"If you get to those levels of success and you have your integrity [like Sheeran] that's a different story but for him it was just false and it wasn't what he needed or wanted or thought he wanted. Sometimes it takes losing things or getting there to realise what you really really want," she concludes, channelling another band altogether.
The Mamma Mia! Here We Go Again star has developed a penchant for music-themed films of late and reveals that in an ideal world she would love to portray the late rocker Janis Joplin on screen.
Back in Boyle and Curtis' brave new world though, the writer acknowledges they "didn't go deep" in analysing exactly how Lennon, McCartney and co "probably changed our whole culture", bending their story towards "youth and love and joy" instead.
"I hope people won't think it's a maudlin thought on mortality and music," stresses Curtis, "because it's in fact a galloping romp with some childish jokes and a lot of teasing of Ed Sheeran."
Yesterday is out in UK cinemas on 28 June.