“Sir Gawain and the Green Knight” is a 14th-century narrative poem set in the Arthurian age. One of my all-time favorite books, it tells of a brave and noble warrior on a likely-fatal quest — but no one knows who wrote it.

If that anonymous poet could see the new film version of his work, I suspect he’d be glad his name is not attached.

Since the book is short, simple and stuffed with florid Medieval description, I expected writer-director David Lowery (“A Ghost Story”) to change it up for “The Green Knight”; but I didn’t think he would tear the tale to shreds.

Here’s what Lowery was working with:

One Christmas, Gawain responds to the challenge from a giant green horseman who’s ridden up to the Round Table: Any knight can have a swing at the visitor with his massive axe — but one year later, the hewer must ride to a distant chapel and let the green man take a similar stroke.

Gawain handily beheads the challenger, hoping that will take care of it. But then the giant grabs his own head off the floor and rides away — after reminding Gawain of his pledge.

A year later, the intrepid hero sets out, winding up at the castle of gracious lord and his randy wife, who seems determined to seduce the knight. So now not only must Gawain complete his deadly trip to the chapel — which is nearby — but also honorably resist the wife’s advances.

Can he do it?

Lowery’s disappointing answer, vastly different from what’s in the book, left me in such despair that I didn’t even want to write this review.

The script seems to aim for the humility espoused in the ancient narrative, but it jettisons Gawain’s principled grace and courtesy. Even sadder is the loss of the author’s delicate humor, his charity toward failure, his sense that Gawain’s fears and foibles are not tragic, but merely a much-needed reminder that no one is perfect.

Here the idea seems to be that no one is good — that rottenness is everywhere (represented by green, as Lowery would have it); it’s even in Gawain, who’s something of a drunk and a coward — and not much of a warrior.

Nonetheless, up to the disgusting bedroom scene, I was actually enjoying the film — even though Lowery added several surreal incidents to Gawain’s journey. The movie is beautifully filmed, with excellent music, convincing dialog and solid performances by Dev Patel (Gawain), Joel Kinnaman (the lord) and Alicia Vikander, inexplicably cast as both Gawain’s girlfriend and the lord’s wife.

With strong direction, “Knight” is by no means a stupid or careless picture; but by forcefully dragging its ethos into the modern era, Lowery has simply wrecked this story for its fans.

And in case you think loyalty to that original is clouding my judgment: I took along a friend who knew nothing about the book.

He didn’t like the movie either.