In theaters this weekend, a creeping crown lands from the sky upon a man’s head and consumes him in fire. Giants striding a canyon refuse calls for a ride. And a tree asks to play a game, and if there’s a lesson to be learned, it’s maybe just ... don’t do that?

Modern livin’ doesn’t mean the old fables are done with us.

Sometimes words fail to illustrate a beautiful thing, which is unfortunate in these precise circumstances. Much like Dev Patel’s Sir Gawain in new A24 fantasy-drama “The Green Knight,” I will undertake a futile quest anyway, with my own hope of some higher reward that I won’t even be around to enjoy: getting you to see this weird-as-hell and gorgeous movie.

Director/ writer/editor/producer David Lowery’s film adaptation of the 14th-century poem “Sir Gawain and the Green Knight” is as haunted as any horror movie artifact, be it a killer VHS tape or a cabin in the woods. It’s not a matter of frights here — the film just leaves you with feelings you can’t put names to even days later.

We’re all in good company: In his introduction for a new Penguin printing of the poem, Lowery writes that “when you watch the finished film, you will see evidence of my … journey through a text I did not thoroughly comprehend. … There is, to put it mildly, a whole lot going on here.”

For those not quite up on their Middle English literature — I am not — the timeless tale written by an anonymous author concerns King Arthur’s nephew, Gawain, and a chivalric hero’s journey. Lowery’s telling finds the restless young man one Christmas, moving from a debauched night in Camelot with his would-be lady of less than noble station (Alicia Vikander) to a holiday feast at the round table. Surrounded by legendary knights with legendary stories to their names, royal Uncle Arthur asks Gawain to spin an entertaining yarn of his own.

“No tales to tell yet,” Gawain replies. Fate, in fine mythic fashion, literally knocks at the castle door in the form of the Green Knight, a mystical behemoth both monstrous and gallant, his body a twisted mass of bark and branch. The interloper challenges the whole court to a Christmas game. Whoever strikes him will get to take the fabulous axe he rode in with, but

that person also must meet the Green Knight in a green chapel a year hence. There, the jade giant will be able to return the earlier blow in equal measure.

Gawain, seeking the honor afforded a hero of King Arthur’s court, takes up the game. He beheads the Green Knight, but a lack of a noggin is more of a flesh wound for enchanted forest warriors. In retrospect, that tracks; our guy is built like a bookshelf. The mysterious man grabs his head, promises to meet Gawain again next Christmas and rides off laughing.

That’s the premise, like the premise of “The Seventh Seal” is that two guys plays chess.

As if he was one of Morgan Le Fay’s coven, Lowery spins moody sorcery in “The Green Knight,” casting all the glamor of a classic knight’s tale — which is plenty to enjoy on its own — while hiding daggers and diamonds behind those dazzling illusions. And they’re really dazzling.

The original tale of “Sir Gawain and the Green Knight,” being old and mysterious, is often mined for symbolism and thematic gold. Lowery puts on his spelunker’s hat and goes for it. For one: The color green itself is both a force for life and one that portends decay both spiritual and physical. Early on, a letter from the Green Knight possesses Queen Guinevere, consuming her from the outside in with a ghastly viridian glow. Later, a stunning overhead shot of an emerald-hued canopy bed suggests a place of healing and, crucially, of tempting passion. Of course, the ominous, verdant gentleman of the title represents a whole undergrad thesis’ worth of metaphor.

For a story so married to color, it follows that “The Green Knight” would be a stylistic coup. Sweeping landscapes — Lowery and co. shot in Ireland — isolate Patel’s would-be knight on his fateful journey. Long stretches without dialogue let the strings in Daniel Hart’s score choke the air out of your lungs. And, hey, period costume lovers: designer Malgosia Turzanska’s Arthurian stunners have just the right touch of the occult for this tale, which does feel like Lowery suffused it with devilry in every frame, just waiting to claim our hero Gawain.

And as that legendary knight (or midknight, in this story), Patel gets the kind of all-eyes-on-him lead role he needs, deserves, slays. A perennial “next big thing” whose IMDb page is studded with projects where his mere presence classed up the joint, the actor is treated by Lowery like the on-screen hero he is. He’s rakishly hunky, he’s hopelessly impotent, he’s anxiously brave, he’s reluctantly noble. “The Green Knight” weaves a million hidden threads into its tapestry; Patel could take the loom apart and put it back together himself.

He leads his own noble order. Vikander juggles two roles — a fine, hallucinatory flourish by Lowery — with skill. Joel Edgerton brings jovial menace to his part as the lord of a manor who takes Gawain in, and Erin Kellyman, recently miscast and doing thankless work as the morally complex antagonist in “The Falcon and the Winter Soldier,” has a fun turn as a spectral, headless saint. (Lotta heads falling off in this joint.)

But for my money, it’s Sarita Choudhury as Gawain’s mother — the king’s sister, and certainly more than she seems — who does the most impressive support work in a role with few words but near-constant looming power. Where her goals lie, exactly, is one of the film’s greatest mysteries.

Out toward the green chapel, mysteries like that grow thick as moss. “Is it wrong to want greatness?” a character in the film asks. It’s one of many existential puzzlers — about honor, about purpose, about self — that “The Green Knight” hexes you to carry into the lobby like an axe you just took off the hands of an elm that likes to monologue.

The journey’s an old one, but I reckon you’re still on it, too.

“THE GREEN KNIGHT” Grade: A

MPAA rating: R ( for graphic nudity, violence and some sexuality)

Running time: 2:10