It is the film that never was – an unlikely sci-fi musical about aliens dreamed up by Paul McCartney half a century ago. The aliens would have landed in a flying saucer, but the project never got off the ground.

Now the former Beatle’s treatment for the film – and an expanded version by the American sci-fi writer Isaac Asimov that McCartney turned down – have been unearthed in a US archive by the authors Allan Kozinn and Adrian Sinclair, while researching a forthcoming book.

The treatment’s discovery is revealed in The McCartney Legacy, Volume 2: 1974-80, published by HarperCollins on 10 December.

After his success with the Beatles, McCartney formed Wings, which included his wife Linda and the former Moody Blues guitarist Denny Laine, and became one of the bestselling bands of the 1970s. Their hits included Mull of Kintyre, a melancholy ballad that outsold every Beatles single in the UK.

The Fab Four had made several films, including A Hard Day’s Night, and McCartney wanted his new band to star in one. He came up with a story about a band of aliens who arrive on Earth, morphing into the members of Wings before challenging the real Wings musicians.

Spanning almost 400 words, his treatment began: “A ‘flying saucer’ lands. Out of it get five creatures. They transmute before your very eyes into ‘us’ [Wings]. They are here to take over Earth by taking America by storm and they proceed to do this supergroup style. Meanwhile – back in the sticks of Britain – lives the original group, whose personalities are being used by the aliens…”

With a working title of Five and Five and One, it had a loose plot outline with dialogue fragments and plot turns seemingly intended to incorporate some of McCartney’s new songs.

Among its eccentric details is an apparent reference to Laurel and Hardy. He wrote: “Onboard the boat they are feeling depressed – huddling leather-jacketed in the corner alcove of the dining room. ‘Another fine mess you got us into Stanley,’ says one of the group.”

In 1974, hoping to collaborate on a script, McCartney went to New York to see Asimov, who was too scared of flying to come to London. McCartney told Kozinn: “He can imagine himself into far-off galaxies, but he wouldn’t get on a plane.”

Asimov, whose Foundation novels are considered among the greatest sci-fi books ever written, expanded and reshaped McCartney’s treatment into a five-­page version. He turned the aliens into “energy-­beings” from a dying planet who want to occupy, rather than clone, the Wings musicians, while being incapable of understanding human emotions such as love.

He wrote that they communicate through “thought-waves”. When they hear music being played, they are “strangely affected” and “decide that they must use the musical key to unlock human emotion”.

Sinclair described the treatment by Asimov, who died in 1992 aged 72, as “a love conquers all tale, in which mankind escapes alien conquest and inherits the universe”.

He spoke of his astonishment that these film treatments have been overlooked among Asimov’s papers: “They’ve just been sitting there.”

He said: “Paul’s treatment reads like something Paul and Linda cooked up while they were smoking something particularly potent … the Laurel and Hardy reference is really bizarre. You can’t really comprehend what Asimov would have made of that.”

Kozinn, who was a music critic and culture reporter for the New York Times from 1977 to 2014, said: “We actually have the correspondence between McCartney and Asimov. We’ve got McCartney’s treatment and we’ve got what Asimov did with the treatment, which McCartney didn’t particularly like.

“By early 1975, the project was abandoned. ‘Nothing ever came of this because McCartney couldn’t recognise good stuff’ was Asimov’s succinct take on the pair’s failed collaboration, scrawled across the first page of his 1,800-word treatment.”

Discussing the excitement of seeing the actual document, Kozinn said: “What’s interesting is seeing what McCartney’s original idea was and how a science-fiction master like Asimov would try to improve it – and the fact that McCartney turned it down.

“We’re talking about Asimov, for God’s sake!”

The new book follows the success of Sinclair and Kozinn’s The McCartney Legacy, Volume I: 1969-1973, the first of five planned volumes on McCartney’s post-Beatles life and career, which was described by one critic as “exhaustively detailed”.

For Volume 2, the writers spoke at length with four former members of Wings, including the drummer Geoff Britton – who has not given an in-depth interview since 1977 – as well as producers and recording engineers, among others who worked with McCartney