Sir Gawain, [Dev Patel], wakes up in a brothel to find his girlfriend Essel, [Alicia Vikander], asking him to go to church, but he doesn’t want to. Gawain also lives with his mother, [Sarita Choudhury].
It is Christmas Day. King Arthur, [Sean Harris], hosts a feast with the other knights and Queen Guinevere, [Kate Dickie]. Arthur asks Gawain to revel him with stories, but Gawain says he has none to tell. Guinevere simply says he has none to tell YET. The feast is then interrupted by the Green Knight, [Ralph Ineson], a large tree-like figure who rides in on a horse. He issues a challenge to everyone there to approach him and strike a blow. Whoever does so must find him in one year to have the Knight return the same blow at his Green Chapel. Nobody but Gawain takes up the challenge. Arthur even allows him to wield Excalibur for the deed. The Green Knight lays his axe down and bows to openly allow Gawain to take a swing. He chops off the Knight’s head, and the body picks the head up to remind Gawain to find him in one year.
The full year nearly passes, with Gawain getting ready to go on his quest. Essel tries to talk him out of it so that he may stay with her and they may start a family, but Gawain feels it is his duty to complete this task. After speaking to his mother and Arthur, Gawain takes a horse and sets out.
On his journey, Gawain spots a fox that is observing him and later rides through a field full of dead bodies. A scavenger, [Barry Keoghan], approaches Gawain and talks to him until Gawain mentions he is traveling to the Chapel. The scavenger gives him directions, but as Gawain starts to ride away, the man asks for a reward in return for his guidance. Gawain gives him a single coin and continues on his journey. While passing through the woods, Gawain is accosted by two other scavengers in league with the first one. They tie Gawain up and steal his horse and the Green Knight’s axe before leaving him there. Gawain crawls over to his sword and cuts himself free.
Continuing his travels, Gawain stops at a house to rest for the night. He is called to by the spirit of a woman named Winifred, who asks him for the favor of getting her head from out of the lake. Gawain agrees to do so but asks her what he will get in return, and the question appears to offend her. He goes underwater and fishes her skull out before going back to the house. Winifred’s head then speaks to Gawain to apparently warn him about continuing to find the Green Knight. Gawain is also rewarded with an axe similar to the one he was previously carrying.
Gawain resumes the journey the following morning. He passes by a group of female giants walking by. When he attempts to ask them for help, one of them appears to try and grab him until the fox returns and howls a kind of tune to the giant. The giant and her group all join the fox in responding with a similar musical noise.
Gawain finds several mushrooms to eat but they make him sick and cause him to hallucinate. He ends up at the home of a couple, known only as Lord and Lady, [Joel Edgerton] and [Alicia Vikander], the latter who Gawain is drawn to. The two also live with an old blind woman that doesn’t speak. The couple offers Gawain shelter after he tells them about his journey. During his time spent with them, the Lady makes an advance toward Gawain in exchange for a sash that she wears that allegedly protects the wearer from harm. She gives him a handjob and wipes her hand onto his, telling him he is no knight.
Gawain starts to leave when he is found by the Lord, who had previously asked that Gawain give to him what he is given back. The Lord kisses Gawain and attempts to seduce him, but Gawain lies to him to get out of it. The Lord then gives Gawain the fox, which he had captured as game.
Getting closer to the Chapel, the fox then speaks to Gawain and warns him that he will be approaching his doom if he fulfills his end of the bargain. Gawain simply responds by attempting to swing the sword down onto the fox, who then runs away.
Gawain arrives at the Green Chapel and finds the Green Knight in a slumber. After sitting there for over a day, Christmas Day arrives, and the Knight awakens. Gawain lays his axe to the Knight’s feet and prepares for the Knight to return the blow that was dealt to him. Instead, Gawain flinches and asks the Knight if this is all there is. The Green Knight replies, “What else is there?” Gawain then runs away and flees from the Chapel.
Gawain appears to make his journey home while riding a new horse. He makes it back to the ailing Arthur and Guinevere, who lie on their deathbeds. Arthur officially knights Gawain, and he succeeds Arthur as king when he dies. Gawain reunites with Essel and makes love to her, fathering a son with her, but the child is taken away from Essel immediately, and she is left crawling in agony before being left out in the slums. Gawain later marries a new queen and lays with her, but he never removes the sash that the Lady gave him. Years pass, and Gawain appears to be an unhappy king. His son lives to his teenage years and is killed in battle. As he faces his subjects, he sees Essel in the crowd staring at him with disdain. Gawain fathers a daughter with the queen, but they leave him as he is now old and gray, and enemies are approaching his castle. As he is left alone and preparing for defeat, Gawain removes the sash, and his head falls off.
This was just a vision that Gawain was experiencing, as he is still in the Chapel preparing for the Knight to swing his axe. Before he does so, Gawain removes the sash and gives it to the Knight, and then says he is ready. The Green Knight then kneels down to Gawain and gently tells him, “Well done, my brave little knight.” He then playfully runs his finger across Gawain’s neck and says, “Now, off with your head.”
