If you were raised and educated in English, chances are you have at least a passing acquaintance with the tale of Sir Gawain and the Green Knight. This chivalric romance comes up in any class that goes back to “Beowulf,” dips into Chaucer and climaxes with Mallory’s “Le Morte d’Arthur.”

As strange and magical as David Lowery’s “The Green Knight” is, that familiarity reveals it as a fairly conventional — or more conventional than you expect — retelling of the story of an ambitious member of court who accepts a combat challenge from an interloper and has his mettle, his mental state and his honor tested by the “game” the green stranger proposes.

There’s little Terry Gilliam/Terry Jones Medieval whimsy in this film from the director of “A Ghost Story” and “Pete’s Dragon.” This is as quiet as a whispered fireside legend, deadly serious and portentous as it honors the somber tone of the original tale.

Dev Patel is Gawain, a young lay-about aspiring to knighthood in the court of his king (not “Arthur,” an opening narration insists), a place at that king’s (Sean Harris, the movie whisperer) esteemed round table.

Gawain has a lover, a short-haired pixie (Oscar winner Alicia Vikander) who wishes he would make her his “lady,” and marry her.

The aged king and his queen (Kate Dickie) long for Gawain’s knighthood so that he can take his place at the table as the son of the king’s sister. She (Sarita Choudhury) is the conjure woman of the court, “a witch,” in the vernacular of the day. And Gawain?

“I fear I am not meant for greatness.”

Christmas is the day of reckoning for this round table of revelers. A towering stranger bangs open the door, rides in wielding a huge battle axe and states his case. Let one knight strike a blow against him, only if that knight visits the stranger in his Green Chapel one year hence, where The Green Knight is obliged to return the blow.

“Oh greatest of Kings, let one of your Knights try to land a blow against me! Indulge me in this game.”

Only headstrong Gawain accepts the insulting, Christmas dinner-interrupting affront.

“Do not take your pledge honoring this idly,” he is warned.

And even when the Green Knight (Ralph Ineson) lays down his axe and offers his neck, the kid doesn’t smell a trap. He takes the king’s sword and slices that head off.

Which the Green Knight’s body picks up and gallops off with, the original “headless horseman” with the head bellowing a reminder about “NEXT Christmas” as he makes his exit.

Gawain has a year to consider the consequences of his haste, the “bravest of the brave” fame it generates (including a Punch & Judy show in his honor), the prospects of maybe marrying better than his beloved Essel (Vikander) and the grim payback awaiting him after a long journey north come next Christmas.

He hadn’t reckoned on the (literal) green stranger not dying of beheading.

The Green Knight himself is leafy, woody and flowering, so much so that you half expect his introduction to be “I am Green Groot.” His underreaction to his beheading brings John Cleese’s “a mere flesh wound” (“Monty Python and the Holy Grail”) to mind. But again, this is more “Excalibur” than Monty Python.

Lowery’s film stakes its claim to “exceptional” in the second and third acts, a lengthy quest to the north where “a small kindness” to a battlefield scavenger (Barry Keoghan) is repaid with treachery, a mysterious “St. Winifred” (Erin Kellyman) requires his service and a noble couple (Joel Edgerton and Vikander again) offer hospitality, with a catch.

The quest is shrouded in shadows and fog, with tension and dread building via long tracking shots of Gawain crossing treeless hills and moors, with the last forests either being chopped down or full of menace, hiding highwayman.

Patel makes a sturdy, sensitive Gawain, someone who lets us see the hard lessons he’s learning with his face and eyes. “Green Knight” is a film of few words in that regard, and all the richer for it.

And thanks to a muddy, gloomy glorious Dark-Ages-on-a-Budget look and the almost heartbreaking pathos Patel brings to each “lesson learned” moment, it works. If you want to know why this fable endures, Lowery’s film makes that case better than any English lit class ever could.

MPA Rating: R for violence, some sexuality and graphic nudity

Cast: Dev Patel, Alicia Vikander, Joel Edgerton, Sarita Choudhury, Sean Harris, Kate Dickie and Ralph Ineson.

Credits: Scripted and directed by David Lowery. An A24 release.

Running time: 2:10