Usually with a good murder mystery, you realise at the dénouement that all the clues you needed were left in plain sight all along. But fair play to Rian Johnson: Glass Onion manages to tell you pretty much everything it’s going to be about and what’s going to happen just through its title.

He landed on it as a metaphor pretty early on. “I literally got out my iPhone and searched my music library with the word ‘glass’,” Johnson told Netflix earlier this year. “There's got to be some good glass songs. I was like, 'Oh, is it a glass fortress? Is it a glass castle? Is it a glass man?' The first thing that came up, because I'm a huge Beatles fan, is ‘Glass Onion.’”

Hence the huge dome much of the action takes place in, a see-through stage for a story about a man with nothing inside of him.

That Beatles tune that lends its title and rounds things off on the credits is more than just a nice nod though. It’s a key to unravelling this onion. Johnson has said he “was always surprised, when I was showing the script around, how many people didn’t know it was a Beatles song,” so let's do a little background.

It’s off the White Album, the one where they were all getting a bit sick of each other and what other people thought the Beatles should be. John Lennon’s ‘Glass Onion’ cannibalises what the band had become for some fans: less four increasingly tetchy Scousers with a neat line in moccasins than living embodiments of the way the new post-revolutionary Western society would be built, their songs runes to be read. Lennon’s response was to take the piss.

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‘Strawberry Fields Forever’, ‘Fixing a Hole’, ‘I Am the Walrus’ and ‘Lady Madonna’ all get a shout out in ‘Glass Onion’. There are lots of musical gags, nods and winks including, after a line about ‘The Fool on the Hill’, possibly the only known example of a sarcastic recorder solo. “Here’s another clue for you all,” Lennon spits. “The walrus was Paul”.

That line was thrown in “just to confuse everybody a bit more,” Lennon explained later. “It could have been ‘the fox terrier is Paul’. I mean, it's just a bit of poetry. I was having a laugh because there'd been so much gobbledygook about [Sergeant] Pepper – play it backwards and you stand on your head and all that.”

It’s almost gleefully anti-meaning: building layer upon layer of stuff begging to be read into, interpreted and sifted for importance, but snickering at anyone who does.

And just as ‘Glass Onion’ is a Beatles song about the songs the Beatles sang, Glass Onion is a murder mystery about murder mysteries. Like Knives Out, it picks apart some of the genre’s most hack moves: the secret twin, a faked death, and, most of all, the evil mastermind with an eye for culture. That final exchange between Birdy and Blanc about Miles’ plan sums it up.

“It’s so dumb it’s genius,” she says wonderingly.

“No,” bellows Blanc, “it’s just dumb!”

preview for Glass Onion: A Knives Out Mystery - Official Trailer - (Netflix)

Miles likes to think of himself and his fellow Disruptors as something like a Beatle-esque cabal of future-moulders. (He’s even got hold of the guitar Paul McCartney wrote ‘Blackbird’ on, which carelessly tosses it to the ground with a clonk.) They’re cultural gatekeepers of exactly the kind Lennon was ranting about, people who were certain they had the keys to the future but who were unaware of the fact they’re the butt of the joke.

They need him to be that genius he presents himself as, and the fact that they willingly look past the obvious bullshitting and confidence tricks to further their own careers is their undoing. Miles’ Glass Onion explodes and takes all the Disruptors with it, wrapping the fate of all of them together despite their last-minute conversion to Helen’s cause.

The song, like the story of Glass Onion, is about building something which looks and sounds complicated, but which is almost daring you to point out that you can see straight through it, and that its centre is hollow. Here’s another clue for you all. The walrus was Paul.