If the original Sir Gawain and the Green Knight isn’t quite as polished as most Arthurian stories, it was written long before most of them - sometime in the late 1300s.
But it’s the blueprint for many subsequent quests – the flawed hero, the initial simple task, that gets fatally complex as it goes on.
The movie Green Knight was written and directed by David Lowery, who made the extraordinary A Ghost Story a few years ago, that featured Casey Affleck under a sheet for most of the film.
This one opens on our hero Gawain waking up in a brothel on Christmas Day.
Gawain is the under-achieving nephew of King Arthur. And after church on Christmas Day he goes to the court presided over by Arthur – the always slightly creepy Sean Harris – and his wife Guinevere – the equally edgy Kate Dickie.
Tell us a Christmas story, demands the King. But Gawain has none to tell, he says.
Be careful what you wish for. The door bangs open and in rides a Green Knight with a challenge.
He invites anyone present to take their best shot at him. The catch is that in exactly one year, the Knight should be allowed to do the same.
Like an idiot – but if he didn’t there’d be no story – Gawain volunteers, and seems triumphant. But only briefly. The Knight revives and tells him to meet him in a year at a remote green chapel.
Gawain panics, but remembers - it was just a game, wasn’t it?
But in chivalry there’s no such thing as “just a game”.
So Gawain takes his leave from his Uncle the King, his mother the witch Morgan Le Fay, and he heads off with, as they would say 500 years later, a bad feeling about this.
The Green Knight is similar in tone to A Ghost Story. It’s slow and meditative, rather than hurtling off in all directions.
But it follows the path of all Arthurian quests, as the hapless Gawain meets with various adventures, often coming off badly.
He is set upon by thieves, led by sinister Irish actor Barry Keoghan. He meets a Lord with a secret agenda, and a wife who looks strangely like Gawain’s girlfriend back at the brothel.
The fact that they’re both played by Alicia Vikander is clearly significant.
There are strange, naked giant women and talking foxes. And at journey’s end there’s the terrifying Green Knight, who seems to be made of ancient wood.
In other words, you won’t be bored when you watch it on Amazon Prime – I’m afraid the tag at the end of the trailer “In theatres this summer” was a little optimistic.
It helps that Gawain himself is played by someone who’s no stranger to the quest movie.
Dev Patel made his name in Slumdog Millionaire, where his quest ended on a TV quiz show, then went on to play Dickens’ rags to riches hero, David Copperfield.
Patel’s Gawain is all-too human, trying his best to do the right thing. But it’s interesting where the film diverges from the original.
In the poem, it all turns out to be a Christmas game after all. No harm done.
But the movie Green Knight offers alternatives that question the ideas of happy endings and doing the right thing.
It’s thrilling, peculiar, frustrating – like watching someone inventing the whole idea of popular stories. Which in a way it is.