Image copyright Bob King Creative Image caption Olly Dobson plays Marty McFly and Roger Bart is the Doc in Back To The Future: The Musical

More people want a new Back to the Future film than want a new instalment in any other franchise. But one of its creators says doing another movie would be like "selling your kids into prostitution" - so it's been rebooted as a stage musical instead.

Walking though the Manchester Opera House foyer a week before the first performance of Back to the Future: The Musical means picking your way through piles of props and kit that are waiting to be slotted into place before opening night.

A skateboard and some of the Doc's scientific equipment are lying around, and a crew member walks past carrying what look like dancers' 1950s dresses. The components of the Doc's nuclear-powered flux capacitor are probably spread around somewhere.

A DeLorean car is on stage, swimming in coloured lights and dry ice.

"Avert your eyes," says the theatre manager, half joking, as she escorts me to a backstage room to interview the team behind the show.

Image copyright Getty Images Image caption Michael J Fox starred in the original film trilogy

In the room, distant strains of the films' familiar score leak through the door, as do futuristic whooshing sound effects. "That's the DeLorean flying out right now," composer Alan Silvestri notes.

Thursday's first performance will mark the end of a 12-year journey to bring one of the best-loved films to the stage. Another journey will start - the show is set to go to the West End after Manchester, and then perhaps Broadway.

It is an adaptation of the original 1985 Back to the Future movie, which starred Michael J Fox as Marty McFly, who finds himself stuck in 1955 and tries to make sure his own parents hook up.

New songs

"It's the same story of the movie," says Bob Gale, who has scripted the stage show and co-wrote the movies. "But there are things that you can do and can't do on stage that differ from cinema."

So in the show, Marty plays more music, and new songs take us deeper into the characters' emotions and back stories. But some of the action (like the skateboard chase and the gun-toting Libyan terrorists) has been changed. And, sadly, there's no Einstein the dog.

Image copyright Phil Treagus Image caption Christopher Lloyd (left) has handed over the keys to the DeLorean to Roger Bart

But they could not possibly stage a Back to the Future show without recreating perhaps the most technically challenging bit of the story - the DeLorean's high-speed time travel.

"Eighty-eight miles an hour, clock tower, lightning - oh, yeah. One point 21 gigawatts. All that is intact. Absolutely," Gale says.

"How do we do it? I'm not gonna tell you that! Good magicians never reveal their tricks."

In a survey by The Hollywood Reporter in November 2018, more people said they would watch a new Back to the Future film than a new instalment from Harry Potter, Star Wars, Toy Story or any other major franchise.

"Lots of people were clamouring, 'Why don't you guys do Back to the Future part 4? Why don't you do a reboot of Back to the Future?'" Gale says.

'The wrong thing to do'

But he and Robert Zemeckis, director and co-writer of the three films, had it written into their contracts with Universal that no new film could be made without their say so. Studio bosses have tried their best to persuade them.

"All the time. All the time," Gale says. "'What can we do to convince you guys to do this?'

"We said, 'Nothing'. 'You'll make a lot of money.' 'We already made a lot of money.'

"You know, you don't sell your kids into prostitution. It was the wrong thing to do. We put 'The End' at the end of part three.

Image copyright Getty Images Image caption Michael J Fox appeared at a Parkinson's disease benefit event in November

"Plus Michael J Fox isn't in the shape to do a movie, and nobody wants to see Marty McFly having Parkinson's disease, and nobody wants to see another actor playing Marty McFly if it's supposed to be a continuation.

"We've already seen the Star Wars movies and Luke Skywalker is an old man." He grimaces. "That can be a little bit painful, right?

"We learn from the fact that so many studios have gone back to the well on some of their franchise properties too many times, and the audiences are disappointed and say, 'Oh my God, they ruined my childhood.'

"We don't want to ruin anybody's childhood, and doing a musical was the perfect way to give the public more Back to the Future without messing up what has gone before."

Image copyright Getty Images

In the stage show, British actor Olly Dobson is Marty McFly, while Tony Award-winning US star Roger Bart is Doc Brown, complete with the 'mad scientist' hair that Christopher Lloyd sported in the films.

The iconic songs from the original films - like The Power of Love by Huey Lewis and the News and Chuck Berry's Johnny B Goode - remain. They are joined by a suite of new numbers written by Silvestri, who composed the film scores, and the multi-Grammy-winning Glen Ballard.

Gale and Zemeckis enlisted the pair to work on the music 12 years ago. "We took this iconic movie and really tried to find the elements of the story that made sense on stage," Ballard says.

"We were taking the Hippocratic Oath approach - first, do no harm, especially to these characters and story that everybody knows and loves."

Image copyright Getty Images Image caption Alan Silvestri, who composed the score for the films, has worked on the musical

Silvestri believes the music draws us further into what is a "deeply emotional" story, and the lives of characters the audience can relate to.

"There's the quintessential bullied kid who is transformed. The bully is transformed. Marty is a great adventurer, it turns out. There's great love and friendship between Doc and Marty," he says.

"It helps because they're so clearly painted in the original film that you can go for big emotion and you can go right for the inner life of these characters."

While the musical's team are not "slavishly" recreating the film, Bob Gale hopes there is enough to make it work on stage and satisfy fans' hunger for something new from Hill Valley.

"There are lots of things that are very much the same. But there are things that are different," he says.

"And we hope we've hit the right mix of that so that people will say, 'I came here to see Back to the Future, and I saw Back to the Future.'"

Back To The Future: The Musical is at Manchester Opera House from 20 February-17 May.