3 stars.

With or without a head on his shoulders, the Green Knight who disrupts a feast at King Arthur's court is an enigmatic character of English myth and legend. What did he want? What did he mean in the great scheme of things? And what about the colour green?

This new film, a contemporary take on Sir Gawain and the Green Knight, a 14th-century medieval fantasy poem of the oral English literary tradition, authored by "Anonymous" has taken some key plot points out of the narrative but offers little in their place.

The sudden appearance at Christmas of a gnarly Green Knight at the court of King Arthur kicks the film off to a promising start. Camelot was in a rollicking mood until a green giant, not in the least jolly, suddenly appeared in the doorway announcing he wanted to play a game. It went like this: the king or a knight of the roundtable could smite the giant knight anywhere on his body, as long as the favour could be returned within a year.

To rid the court of this pesky malevolence and spare his king, the best of Arthur's knights, a young man of the highest calibre, steps forward and does the deed, cleaving the Green Knight's head from his body. But the Green Knight (Ralph Ineson) picks up his head and leaves, reminding all that the most chivalrous and excellent young Gawain (the charming Dev Patel), must honour his word.

However, Gawain is not quite the upstanding character he is supposed to be. The gradual undoing of a man of impeccable morality was the point of the original poem and the version of Gawain we find here is one of many hints that the film is keen to show the downfall of the Camelot ideal, rather than examine the reasons for the decline.

Written, directed, edited and produced by David Lowery, The Green Knight has been fast and free with its source material, on this and other points. Few in the audience besides former students of English lit, like me, may care about the loss of key content, like fiddling with characters like Morgan Le Fay, but the point of the original has been lost somewhere between the moors where scavengers roam and the castle where Gawain undergoes a trial by seduction.

Lowery's recent A Ghost Story, set in a haunted house with a ghost wandering around in a sheet, was an odd and dissatisfying experience. Here, however, the filmmaker, his cinematographer Andrew Droz Palermo and costume and set design have certainly achieved a great look, using the Irish landscapes to wonderful effect, relying modestly on CGI.

Dramatic achievement is another story. During the film's first half, The Green Knight is reasonably compelling, but the narrative threads get lost among the gorse on the moors, in the tenebrous forests and ruins dotting the landscape.

The Green Knight never delivers on its early promise by maintaining the level of tension we are set up for. As the solitary knight traverses the landscape, he is robbed of an axe and girdle guaranteeing his safety, he takes up company for a while a solitary fox. Perhaps the creature signifies Gawain's growing wisdom. Who knows? All the while, choral voices on the soundtrack become increasingly insistent.

The film's weak script never fully allows for the subtlety of the trial that Gawain undergoes within the castle of the Lord, Joel Edgerton in a welcome appearance, and his Lady, Alicia Vikander, who was in early scenes as Gawain's love interest in a bawdy house.

While the poem is explicit about how his noble hosts test him on three separate occasions, in the film the tension of this long episode is cut short and the ending has been rewritten so it loses the point entirely

I will always argue that the screen is a very different medium from the page, and that film should be judged as a different medium, but this film version fails to offer something new and meaningful in place of what has been excised. So, as King Arthur's finest and most virtuous knight seeks his destiny across the land, how he learns of the importance of honour, truth, and wisdom, or of being a good and just man, gets the abridged treatment.