StarSolidStarSolidStarSolidStarHalf(3.5 stars)

The world is full of all manner of mysteries, according to a character in “The Green Knight.” And the film — based on a 14th-century Arthurian tale of anonymous authorship that is considered a meditation on the notion of knightly honor — certainly bears that out.

Loosely adapted by writer-director David Lowery (“A Ghost Story”), the film includes: a knight who looks like Groot (but is a lot more articulate) and who stands up and rides off on a horse after getting beheaded; a second victim of beheading, St. Winfred, who still happens to have one noggin on her neck when she manifests, like a vision, to the film’s hero Sir Gawain (Dev Patel), as well as a second, severed skull at the bottom of a spring; and a talking fox. That fox, while kind of cute, is nothing like the one voiced by George Clooney in “Fantastic Mr. Fox.”

There are also naked giants in the mist. Other strange and wondrous things occur in this richly allusive, poetic tapestry, which is not always easy to untangle, but it’s worth trying.

And yet the plot is pretty bare bones.

One Christmas, as several knights have gathered for merriment at the court of the king and queen (Sean Harris and Kate Dickie), a figure (Ralph Ineson) appears astride a horse, with the face of a tree, the voice of a broken steam engine and an ax, to issue a challenge. Whosoever lands a blow against him gets to keep the ax but must agree to meet the stranger one year later to accept a commensurate blow in return: a nick on the cheek for a nick, a head for a head.

Gawain — who is not yet a knight but wishes to become one — accepts the dare, and when the Green Knight, as he will be known, kneels, offering his neck, Gawain seizes the opportunity. A head rolls, is scooped up, and the knight rides off with it in his hand, cackling like something out of Washington Irving.

Fast-forward one year. (Or rather, slow-forward. As with Lowery’s poltergeist film, nothing moves very quickly, despite the on-screen title “A Too Quick Year,” introducing a chapter that tracks the nerve-racking period of Gawain’s waiting.) Eventually, Gawain sets off on his quest, is waylaid by thieves, meets the fox, meets Winifred (Erin Kellyman), and rests a few nights with a hospitable lord and lady, played by Joel Edgerton and Alicia Vikander (who also plays Gawain’s girlfriend back home).

Like the source material, this is a story of temptation and lessons. Green, the color not just of life, but of jealousy and rot, is only one of many multifarious symbols here, in a legend that plays out less like a one-to-one allegory than a dream, in which one thing can mean many different things, sometimes contradictory, at once. In legend, Gawain served under Arthur, and Gawain’s mother (Sarita Choudhury) was Arthur’s sister, the sorceress Morgana le Fay. Other than Gawain, though, few names are mentioned here. But there is a Merlin-like wizard figure, and the character of Gawain’s mother appears — or seems to appear, like Vikander’s characters — in more than one guise.

“The Green Knight” casts a magical, and sometimes mist-bound, spell.

Yes, there is a moral to the story. But just as it is left to Gawain to figure out what that is, so too are the takeaways of this film — which include both abundant pleasures and perplexities — left to the viewer to decipher, perhaps long after the closing credits. The yarn that Lowery spins is rich with incident, but ultimately simple. Its enjoyment lies less in the story, but in the marvelous mystification of its telling.

R. At area theaters. Contains violence, some sexuality and graphic nudity. 125 minutes.