As a longtime appreciator of Arthurian lore, I’ve contemplated more than once the challenges facing anyone who would try their hand at adaptation. An artist or filmmaker must attempt to hold together not one, but two, dialectical pairs: the tension between paganism and Christianity, and the transition from the Roman to the medieval age. Failure to strike the proper aesthetic balance inevitably leads to an unsatisfying result. (Just think of Guy Ritchie’s catastrophic “King Arthur: Legend of the Sword.”)

David Lowery’s eerie, slow-burning “The Green Knight” delivers on this unified vision. No doubt this take on the classic tale of Sir Gawain and the Green Knight won’t be to all viewers’ taste, but those willing to fall under Lowery’s spell will find themselves drawn into a mesmerizing world that—to its tremendous credit—never attempts to demythologize or seriously subvert its subject matter.

Readers familiar with the original poem—perhaps most memorably translated by J.R.R. Tolkien—will find all the standard story beats present here. Young Sir Gawain (Dev Patel), while feasting with the Knights of the Round Table on Christmas Day, is abruptly confronted by an inhuman Green Knight, who challenges him to a game: deliver a single blow, and then travel to the Green Knight’s abode a year leader to be repaid in kind, as a gesture of reciprocity. The arrogant Gawain promptly beheads the stranger, only to learn too late that the Knight cannot be so defeated. Laughing all the while, the mighty creature departs, reminding Gawain before leaving that he must present himself the next Christmas to receive the same stroke delivered back to him. Gawain’s days, in short, are numbered.

Honor prevails: Gawain leaves the arms of his lover Essel (Alicia Vikander) and sets out on a quest across England’s fog-shrouded moors, confronting bandits and saints and giants in the process. At the end of his journey lies the Green Chapel where his fate will be decided—along with an enigmatic husband and wife who seem strangely familiar.

(Mild spoilers for the movie ahead, though these don’t really count if you’ve read the poem.)

Lowery’s last film, the sad little metaphysical romantic drama “A Ghost Story” (sorry for the four adjectives, but they’re all apposite) concluded with an extended flash-forward, drawing on the motif of the “eternal return” to demonstrate that, after an infinite span of time, all events inevitably repeat themselves. Here, Lowery uses a similar technique to express a fundamentally different theme: the possibility of alternative futures, rather than of cosmic repetition. In the instants before the Green Knight’s axe descends on his neck, Lowery’s Gawain imagines all those those things that would happen if he simply fled the chapel and returned home: succeeding Arthur on the throne, consolidating power by pushing aside his loved ones, and finally witnessing the fall of Camelot in a storm of blood and fire. And Gawain, crucially, rejects that path, remaining resolute as the Green Knight towers over him. To live dishonorably is, in short, no proper life at all.

And then the movie ends, moments after the Green Knight rumbles “off with your head.”

Those familiar with the poem know what happens next: the Green Knight turns out to be the lord of the nearby manor, who devised the whole experiment as a test of Gawain’s moral mettle. But Lowery leaves the matter unresolved, ambiguous—a kind of Pascal’s Wager for the knights’ code of chivalry. Gawain’s choice is vindicated not because of its good outcome, but because of its intrinsic virtue.

This is not a stylistic choice that many—even most—viewers will understand or appreciate. (Almost everyone in my theater was completely dumbfounded by this conclusion, and I heard a lot of grumbling on the way out.) And yet it brings to the surface an essential truth of life: all of us, at least on the level of the immanent, must make moral decisions against a backdrop of profound uncertainty. We do not know in advance what the decisions we make will lead to, or whether we ourselves will survive the process—but part of being a moral agent as such is the need to make such choices nevertheless. The narrative structure of medieval romance leads the reader of the poem to presume that all things will ultimately work out, and thereby to impute that awareness to the story’s characters—but “here below” in the real world, and so too in Lowery’s adaptation, the future is experienced as clouded and doubtful.

In so framing Gawain’s tale, Lowery manages to make this old tale “relevant” in a genuinely existential way, tapping into eternal truths rather than drenching his story in biting irony or sociological critique. It is this element, I think, that will make “The Green Knight” endure for years to come, when many other Arthur adaptations are long forgotten. There is a reason this particular legend has persisted over the centuries, through countless social upheavals and ideological revolutions, and Lowery’s film successfully channels that ethos.

Those unwilling to sink deeply into this film’s lush tapestry won’t find much to like here (this is not a film, for instance, that can be watched with one eye on one’s phone)—but more patient viewers will find themselves richly rewarded.