This is not as much a review of the movie but an attempt to clarify it, as its story seems to have been widely misunderstood. I will first summarize the main events as told in the original tale, and then compare them as told in the movie. It will be found that the movie inverts the original tale in a highly innovative manner.
In the original tale, Sir Gawain is an honorable knight who unknowingly gets in the way of a witchcraft ploy by Morgan Le Fey (not his mother!) to gain power over the Arthurian throne within a year's time.
When the Green Knight, summoned by Morgan, appears in court on Christmas day and issues his challenge, it is King Arthur who is the target. Gawain courageously takes Arthur's place in confronting the Green Knight in order to assume the consequence of decapitating the Green Knight a year later for himself: meeting him at the Green Chapel and suffering a similar mortal wound.
A year later, when he is ready to face his end of the game, he maintains his honor throughout, except on one occasion: he breaks his word in an agreement made with the lord of a manor he comes across just prior to reaching the chapel where he is to meet the Green Knight.
He and the Lord had agreed that while he stayed there, each day the Lord would give him whatever he gets while on the hunt, and Gawain would give him whatever he gets in the manor. While the Lord is out hunting, his wife, the lady of the manor, attempts to seduce Gawain, but it only ever leads to a kiss and nothing more. When the Lord comes back, he presents him with the game he caught, and Gawain presents him with a kiss on the mouth, thus keeping his end of their agreement. These events repeat the next day.
However, on a subsequent day, when the lady of the manor offers him a green sash she tells him will protect him against the Green Knight and save his life, he keeps that from the Lord and thereby breaks his end of the agreement, while the Lord presents him with a fox he hunted.
Gawain then goes to face the Green Knight, and assumes the position to be decapitated as agreed, but flinches at first. Eventually, he gains his composure and is ready. The Green Knight lifts his axe, and it comes down just by Gawain's neck, imparting merely a glancing wound.
It turns out the Green Knight is actually the Lord (transformed by Morgan), and the glancing wound was only because of the slight breach in Gawain's honor when he failed to disclose the receipt of the green scarf, otherwise the axe would have missed his neck completely, as Sir Gawain's honor is otherwise spotless, and the Green Knight's axe cannot harm an honest honorable man.
Now to the movie (Lots of Spoilers):
Morgan Le Fey is Gawain's mother and wants her son to one day succeed King Arthur on the throne. The problem is that her son falls drastically short of the chivalric ideals of a knight. He spends his days in a whorehouse with a prostitute, imbibing alcohol and doing little else, even on Christmas, and then lies to his mother about it. He is not an honorable man.
So Morgan concocts a plan to use her witchcraft that will help demonstrate to others her son's honor, thus giving him a reputation worthy of the throne.
She summons the Green Knight, and the target of the challenge is Gawain. Her son does indeed rise to the Green Knight's challenge. However, it would have sufficed for Gawain to demonstrate his honor just by nicking the Green Knight and agree to be nicked himself a year later. Indeed, he is indirectly encouraged by his uncle to do just that when told "Remember, it is just a game", but the reckless young man instead chooses to decapitate the Green knight, presumably reasoning that this will obviate the need for him to subject himself to any kind of harm a year hence.
But the Green Knight then gets up, picks up his head and reminds everyone present of Gawain's end of the bargain before galloping off.
In the intervening year, Gawain has still not attained much honor. Among the populace, his encounter with the Green Knight has been spun into a tall tale of courage and honor which bears little resemblance to the actual event.
But as flawed as Gawain is, he does want to attain the chivalric ideal of honor, and so, with encouragement from his uncle, he goes on his way when it gets close to his appointment with Green Knight.
Unfortunately, already at the outset of his journey, his honor is compromised because he obtains "insurance" that nothing will really happen to him, as his mother gives him a green scarf that will protect him against the Green Knight.
Along his journey, he is presented with a number of opportunities to prove his honor, and he spectacularly fails almost every time: when attacked and captured by a group of bandits, he begs for his life like a coward, when asked by a mysterious lady ghost to retrieve her skull, he queries what she can do for him like a haggler, and when the lady of the manor attempts to seduce him, he completely gives in like a lecher.
In the one situation in which in the original tale Gawain's honor is blemished, in the movie he manages to save it: after he has obtained a replacement sash to protect him (the first one was taken from him) and per agreement is obliged to give it to the Lord, he asks to be "unhanded", presumably meaning to be released from their agreement. The lord agrees, and releases the catch of that day instead of giving it to Gawain: a fox which had joined Gawain earlier in his journey. He also kisses Gawain on the mouth, which I interpret as a hint that this story is the inverse of the original tale.
As Gawain comes very close to the green chapel, the fox suddenly turns out to be able to speak and tries to dissuade him from proceeding. The fox's voice is that of his mother!
It turns out that Morgan had been watching over Gawain all along, accompanying him on his journey in the form of the fox. Presumably, she also arranged for him to regain the items he had lost along the way: his horse, the axe and, most importantly, the green scarf that could literally save his neck.
Why did she reveal herself now?
The honorable thing for Gawain to do would be to present himself to the Green knight without his sash, but that would mean certain death for him. Throughout the journey, Gawain had acted reliably in a dishonorable manner, but now, Morgan had just witnessed her son for the first time acting honorably: instead of breaking his agreement, as in the original tale, he asked to be released from it. Might this inspire him to act honorably when he encounters the green knight?
Presumably, this is the fear that compels Morgan to blow her cover, but to no avail: Gawain continues his journey until he meets the Green knight.
He assumes position but flinches. He asks the Green Knight whether that is all, getting in response asked what else there ought to be. This seems to be a nod to remind the audience that in the original tale, there was indeed more, but not here.
He assumes position again, but loses his courage and runs away. In a lightning-fast sequence of events, we see him return to Camelot, assume false honor, succeed King Arthur on the throne, father a son with his prostitute lover, take it away from her and marry a more reputable lady, unsuccessfully seek honor in further conquest, eventually losing his son on a campaign, then his kingdom and finally his head.
This turns out to to have been a vision of what his life will be like if he chooses the way of dishonor as he has almost always done. He decides to change, and removes his scarf before assuming position to be decapitated. During the literally last minute of his life, he becomes an honorable man.
The manner in which the movie inverts the original tale is nothing short of brilliant. I would have rated the movie, which in all other aspects (cinematography, music, acting) simply dazzles, a 10/10 if the inversion and the meanings had been handled more transparently.
The fact that so many people have failed to understand what the movie is about is prima facie evidence of a massive failure of the movie to communicate. Many films are criticized for being too obvious, this movie is too opaque. What a loss for everyone who saw it but did not understand it.
